Getting back my security deposit

[willythree]

My girlfriend and I broke up and moved out of our shared apartment. A rider on our lease required 60 days notice for move out, we only gave like 33 days. But I showed the apt for the landlord and new folks moved in the day after we moved out (and we paid all rent on time and left everything in perfect condition). My girlfriend has now moved into another building owned by the same landlord. The landlord, who is by all accounts insane and dishonest, is now attempting to keep my security deposit from the original apt. My understanding is that a landlord can only keep a security deposit to cover physical damages or unpaid rent. My question is, now that my ex-girlfriend has a new lease in hand, my best leverage seems to be for her to refuse to pay her rent up to the cost of the security deposit they owe me. Is there a downside to this plan (other than pissing off the landlord), or a better way to go about this? Thanks for any advice.