Assuming that Bob is innocent (I know, shocking these days, isn't it?)
Bob should take time off immediately to speak to a lawyer about this.
He should say NOTHING to HR without a his lawyer present, or conferenced in. Not a word to Alice, HR, coworkers or anyone else about this, not even to protest his innocence.
He needs to get a good employment lawyer versed in the laws of his state. His lawyer should then obtain a copy of the company's policy regarding such matters.
Things will likely get unpleasant and aggressive. If this is baseless, Alice will likely lose her job for filing a false complaint, so prepare him for that.
He needs to act fast to protect his reputation. He may be done at this company. A lawyer may negotiate severance and a quashing of the accusation in exchange for not suing the company for a false accusation.
Bob needs to be ready to go to the mat to defend himself. It's going to be ugly no matter what happens.