What can my player do to get advantage with picking a lock (or all locks) without relying on another PC's help action? Is there an item that grants this, or a scenario in which the DM should impose advantage?
The easiest way is a wonderous item that grants the Cat's Grace version of the Enhance Ability Spell X times per day.
You could go with a wand, potion, or a ring of spell storing, but they still rely on a spellcaster at some point. I am assuming the player in question is not spellcaster. Still, it is much less of a drain on their fellow spellcaster's resources.
A magnifying glass gives advantage to all checks when manipulating something small. Perhaps, if the DM is so inclined, a jeweler's loop may help the lock picker.
Perhaps they inherit an exceptionally fine, non-magical, set of lock picks. Or maybe they are magical. Such a thing would surely be pursued by theives, though!
There is the option of spending inspiration, but I hope they don't have too many locks to get through.
As always, the DM get to choose what grants advantage. It can't hurt to start a dialog.
If you have a DM awarded Inspiration point, that acts as having advantage for one roll. If your DM does not award inspiration, I suggest you have a talk with the DM concerning that feature. (DMG 239-241).
There is an answer covering negotiating with your DM for Inspiration here.
Some ideas on suggesting where advantage is appropriate (credit to @Luke)
Previous knowledge of the particular lock (seen the key, unlocked many of this type before)
Situational insight into a weakness of the locking mechanism (Player investigates lock first "a gentle twist of the lock's barrel reveals that there is a greater degree of movement than there should be
been installed with little care or endured mistreatment; either way, this lock would be susceptible to a good, ol' fashion riggling"
Granted, some of the above may be folded into the DC that the DM assigns to a given lock in the first place, but it never hurts to ask/investigate. Discuss with your DM.
Do anything that would, in older D&D editions, provide a positive modifier to the check
5e uses advantage in lieu of modifiers, so achieving advantage on such a task should be fairly trivial in the absence of time pressure. For example, you could:
- research the locksmithing methods of the creator of the lock for hints about weaknesses in the lock's construction.
- approach the lock midmorning, after a full night's sleep, breakfast, and a morning of preparatory exercises.
- use drugs to optimize your mental state
- ask an NPC to use the help action
- entreat the God of Thieves for aid on the task, perhaps with a small sacrifice at a shrine or somesuch
- Use a wedge to maintain torsion wrench pressure without active involvement, so you can pick the lock tumbler by tumbler over a matter of days rather than hours.
- Solicit advice from persons of greater skill at an appropriate venue
- Use thieves' tools manufactured by the same culture group as the security measures.