My bank has a little lobby with a coffee machine where I and a couple others hang out and work. I like to stand up while I work and they have a couple taller tables that work nicely for me.
As long as he's a customer and doesn't seem to pose a problem, they probably don't mind. (And bankers would do something if they perceived a problem, if only by having the security guard hang out very close to the people in question.) If they haven't noticed that a few people work in the lobby, then those people really must not be impinging on anyone there.