There is a chest and the Rogue rolls and detects a trap. A needle trap that would eject into the thief's hand and poison them.
The Rogue's player says that he borrows the Fighters metal gauntlet (which the needle couldn't penetrate) and safely sets off the trap.
Roll for disarm or not?
Depends on the context.
A gauntlet actually made of solid metal, mitten-type, won't be able to open the chest. One that is articulated enough to allow someone to spring the lock (and thus set off the trap) will have sufficient weak points that the trap could still harm them, but the gauntlet will certainly make it harder for the needle to get through, as would the fact that the thief (presumably) knows where the needle would come from. A more common gauntlet, which would be just leather with a coating of metal plates on the outer surface, would not provide any better protection than the leather gloves I presume the thief is wearing.
So, if the gauntlets are good enough to block the trap, they're not going to work to open it in the first place. No roll, doesn't work, but it's a clever idea, try again with something more flexible, or something that works from a longer distance away (e.g., repurpose a crowbar to open the lock from the side.)
If the gauntlet
is articulated enough to do that, then it's almost certainly not solid plates, and thus has weak points on the hands where a dart could slip through, but it'll be difficult. Roll with a bonus (or roll against a lower target number, same thing.)
If the gloves are flexible because they're leather with riveted metal plates on the back of the hand/wrist (which most armored gauntlets would be, especially for low-level characters), then they provide no special protection, roll as usual.