You may subpoena the old landlord to testify at trial or deposition and you can request that any relevant records be brought to the hearing. You must provide 15 days notice to the
BM that you intend to subpoena these records, to give her an opportunity to file an objection with the court. Once you have the landlord's attention, you can negotiate with the BM to stipulate to the reason why she left, in an effort to avoid having to prove the matter at trial.
Whether this information is actually relevant to prove the BM's parenting abilities is highly arguable, especially if it's an isolated incident. I can't analyze further without more facts.
