I wouldn't get involved at all other than being a good witness. Even though I "
know 100%" it's a fake gun, they pay people to deal with this sort of thing. Also, what if I were wrong about the gun?
If the bank isn't concerned enough to station an armed guard(s) on it's property (or the guard present is not intervening), I sure don't care enough about it's money to lift a finger. I'll protect life, not a bank's cash.
Of course this all would change if a really loooooow fixed interest rate were a potential reward.
Kidding.