It sounds like he's testing you. What follows is an in-game option you have for showing him that you aren't messing around; that, when you're DM, each party member will have strengths and weaknesses, and choices have consequences.
Allow the heist, but make it a pain in the butt that results in a net loss to the player.
Tell the player, "Old Man McMageFace isn't your character anymore. He's now just another high-level NPC. If you insist, though, just this once I'll allow a high-level NPC to part with his gear. Normally, said NPC would murder you in your sleep within 24 hours, but I'll even decree that Old Man McMageFace has completely lost his marbles and doesn't care that his stuff is gone. Mask, God of Thieves, appears before your new character, saying that your old character's equipment can be stolen without retribution from said character. The only condition is that you, the player, don't get to utter a word of complaint if it makes the game less fun." If your player is gutsy enough to say "Okay"-- well, read on.
Valuable things gravitate toward owners who can hold onto them
Why do stronger monsters have better loot tables? It's not just about game balance; it also makes for a more believable story. Consider three possibilities for Old Man McMageFace and his Staff of the Magi:
Option 1: Old Man McMageFace can and does defend his belongings
This is what makes sense. He survived adventuring because he had the wherewithal not to do stupid stuff.
Option 2: Old Man McMageFace stopped defending his belongings two years ago
Ask your player if he's sure it's been years since Old Man McMageFace stopped protecting his gear. If "yes", okay, the old wizard lost his magic items three weeks later to some misguided teenagers.
"No problem. We just find those teenagers." Haha, that's funny. They're long since dead, having been killed by the local thieves' guild for the Staff of the Magi. The local thieves' guild, of course, promptly saw a bloody change in management when a vampire took over to get the staff. Then the staff was sold to a prince in a neighboring kingdom, but the caravan was attacked and looted. That was all within the first three months.
To make a long story short, powerful people are willing to kill for such a rare item, so it's been a violent two years. Where's the staff now? Some say it found its way to Athkatla, others say it's at the bottom of the sea, and one guy insists it's now on the moon, with Steve.
Option 3: Old Man McMageFace just lost the ability to defend his belongings five minutes ago
Maybe your player backpedals. "No, actually, I mean he becomes easy to rob when I arrive in Waterdeep."
Fine. Your players' party of nobody's-heard-of-them characters steal an extremely valuable trove of magic from a highly renowned member of the arcane community, including a rare Staff of the Magi. Here's a timeline of events after the robbery:
- 24 hours: this shocking news reaches a Lich. "I've wanted a Staff of the Magi for years!" he mutters.
- 33 hours: The Lich finishes swapping out spells.
- 33 hours, 6 seconds: The level 1 wizard fails a wisdom save against Scrying.
- 33 hours, 12 seconds: The Lich successfully Teleports right next to the party.
- 33 hours, 18 seconds: See below.
- 33 hours, 24 seconds: The Lich Plane Shifts to a demiplane safehouse, Staff of the Magi in hand.
You have a choice as to what spell the Lich casts at the 33 hours, 18 second mark. Meteor Swarm is the Lich's smart pick, but using Wish to duplicate a level 8 casting of Command ("Drop!") will work if you don't actually want to kill the party. In an earlier edit, I said Dominate Person and Power Word Kill, but the Lich would realize that either of those would trigger the Staff of the Magi's spell absorption property.
If your troublesome player is lucky or flagrantly cheats on his dice rolls (don't let him cheat on his dice rolls), but you don't want to kill the entire party, use damage spells with a tight AoE (Aganazzar's Scorcher, Erupting Earth, Thunder Step if you want a Con save, Create Bonfire as a legendary action). He'll drop the staff when he hits zero HP (he also might die; more on that in a moment).
Modify the timeline as needed. Maybe you wait a week for the Lich to learn of the under-guarded staff, so the Problem Player can feel like he got away with it. If you want to build tension, add a couple house rules: First, the wizard gets to know that he just attempted to save against Scrying; second, the rumor the Lich heard was so vague, the Scrying DC starts at 1, but it increases by 1 with each casting (the DC for a Lich would normally be 15 due to the Lich's lack of familiarity with the target). On a successful save, the Lich has to wait 24 hours to try again-- so every day, at the same time of day, your Problem Player is being told to save versus Scrying with no explanation as to why. If you do this, the player might try to argue that the Staff of the Magi absorbs the Scrying. Either tell him it can't ("The targeting is too subtle for you to react to it.") or target one of the wizard's low-wisdom-save compatriots.
Of course, don't feel like it has to be a Lich; lots of level-inappropriate monsters would LOVE to get their hands/claws on a Staff of the Magi, either to use or to sell.
Should you actually do any of the above?
If you wipe out the party out of frustration, it's the Rocks Fall, Everyone Dies trope, but with an absurd amount of embellishment. If the whole thing makes sense as a story, though, that's choices having consequences. YOU are the DM. You can-- and should-- make judgment calls along the way. For example, let's say the Lich knocks the wizard unconscious. Look at your friend. Does he get it? Does he realize that you're not screwing around-- that you'll wreck his day if he tries something as obviously-stupid as trying to transfer magic items from a previous character?
If you need to be mean, the Lich stuffs the wizard's unconscious body (items and all) into a Bag of Holding, then Plane Shifts away. Hand the player a fresh character sheet; advise him on which rule options you use for rolling a new character. Fun fact: Liches need to devour souls to survive, and that doofus wizard with the Staff of the Magi has one tasty-looking soul.
If you don't need to be mean, the Lich merely plucks every magic item from the wizard's body, then leaves. If the wizard died, it's up to the rest of the party whether they'll raise him. If they do, now the wizard owes all this money, so basically his stupid idea will cost him his share of the loot, perhaps for multiple sessions.
This player just really screwed over his old character by declaring that his brain was wrecked. I mean, jeez. Poor Old Man McMageFace. He's got some kind of neurological condition. Make a running gag out of it, but make the gag more sad than funny.
The characters visit Lord Questgiver's estate. Judging by the decorations, they seem to have arrived during some special occasion. Wait, isn't that Old Man McMageFace? Since he lost his equipment and his marbles, he's been doing prestidigitation at children's birthday parties to make ends meet.
The characters visit a sanitarium. Again, wait, what's Old Man McMageFace doing here, and in a straitjacket no less?
The characters enter the vampire's crypt. SHH! Oh, wait, it's Old Man McMageFace? Working as a janitor?
If you don't think feathers have been sufficiently ruffled by the above, pick a major accomplishment of the previous party. Undo it. "Yeah, without the threat of Old Man McMageFace, The Cult of Cliche Evil reestablished itself and managed to resurrect [insert recurring BBEG from the previous campaign]."
In short, choices have consequences, especially when those choices are so obviously not in the spirit of the game.