My former roommate is being hassled by my former landlord and his current landlord. A few months after I moved in, it was found that the apartment manager at the time was embezzling money from the apartment complex. It was found that our account had a negative balance, and we were asked to provide proof of payment. Unfortunately, I made the mistake of paying a month of rent in cash (I worked as a waitress that summer) so I have no proof of that month. However, all other months are covered and in all other aspects my history is fine.
Unfortunately, although I have moved out, my former roommate has not, and is still living at this apartment complex. They are sending him threatening letters regarding his account (apparently this carried over when he re-signed his lease). There is nothing I can do to prove that I paid my account in cash and the apartment complex has NOT contacted me regarding this problem at all. My former roommate is threatening to take me to court, but I DID pay my rent and I really would like to avoid paying it a second time.
What are my legal obligations here and what should I do to remedy this situation?

You are obligated to pay rent under the terms of the lease as far as the complex is concerned. At the moment it doesn't appear anyone is looking at you to make an additional payment.

If they do pursue you, it will be a Small Claims Court matter and the Judge will decide which side of the argument is the most feasible.

I'd do nothing until someone else does something.