In a campaign I recently played, I encountered a NPC that could do everything a lvl 7 Rogue could do, for instance he was able to pick locks but he didn't even roll for it. Later in the storyline, he revealed that he was a bard. My question is: Can a bard pose as a Rogue and have all of the lvl 7 Rogue abilities?
Class names and in-universe terms don't need to be the same
What, really, is a 'bard'? It's somebody who earns their living by entertaining people with music and song. This doesn't require any form of magic, a peasant with a lute and a good singing voice could still make a living as a bard, even though they have no levels in the bard class as a player would see it.
So it's perfectly possible for somebody who has all the skills of a rogue to call themselves a bard without lying about it. As far as they are concerned, they are a bard, it just doesn't match your expectation of what a bard is.
Likewise, it's perfectly possible to find someone who calls himself a 'Songsmith' or a 'Bladesinger' in-universe, while using the game mechanics for bards.
On top of that, you're talking about a NPC here. Even if your DM was talking in game-terms, there's nothing stopping a NPC from having bard AND rogue skills. They do not follow the normal rules for player character building.
From the situation you present, there is nothing that makes me jump. A "rogue/thief" can be of any class. Now does "All the Rogue abilities" mean the bard had sneak attack, uncanny dodge, and other?
NPCs do not have to confirm to exactly the same rules as PCs, it depends how the npc was built.
Was the NPC interesting? If so, then roll with it.
I have a character - Igor - who is a warlock servant of Master Strahd. He has skills with thieves' tool ("To break into morgues") and does a lot of rogue type things: sneak around, deal with traps and locks
A bard can pretend to be a rogue, but they will not have all of the rogue's abilities
Some things can be done
Picking a lock only requires proficiency in thieves' tools, and a successful dexterity check. Getting proficiency in thieves' tools is fairly easy, for example taking the Criminal background. If someone says "I'm a rogue" then proceeds to pick a lock with ease, it's understandable that you would think they are a rogue.
Some things can't be done
While it is easy for any class to pretend to be any other class, there are some things that a rogue can do which a bard can't.
- Communicating in Thieves' Cant
- Hide, Dash, or Disengage as bonus actions
- Extra damage from Sneak Attack
You can always fake it
But, a bard could fake it. They could pretend to communicate in Thieves' Cant and you wouldn't know the difference. They could act as if they took Hide/Dash/Disengage as a bonus action, but actually take them as an action or not at all. They could act as if they are getting sneak attack damage, but really deal regular damage.
You can make your own NPC with all the abilities of both classes
If all else fails, there is always the brute force solution of having them be both. I know this doesn't really fit your situation, but it is possible that a character which is a "7th level bard" has all of the "7th level rogue" features bolted on to them. Ugly and messy, sure, but possible.