Guru--
What about the language of the lease? If it says ALL parties, is it or isn't it ALL parties? If she had signed it and he hadn't, and the landlord didn't think she could afford the place, or didn't think she was a good credit risk, or any of a number of other reasons, would she be able to enforce it and take possession?
If not, then where is the logic? These property management outfits are myriad down here because we have sooo many non-resident property owners. And the PMs are often unscrupulous bullies, as these are, but whether they are jerks or ethical business people with a legitimate disagreement on circumstances, they usually get their way because most renters don't have the time, money or resources to argue with them.
And if he's rented the place again, where is his harm? What are his damages? In Florida, at least, you can't keep a deposit for the hell of it, no matter how inconvenienced or annoyed you are.