Generally speaking it should be handled in the marital settlement agreement. If none then I would say split equally 50/50. Speak to their divorce attorney if you can.
I am a Florida Licensed Attorney and this information is for general information purposes only. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
If the security deposit was paid before they were married then it would be separate property. Even though separate if may have been addressed in the divorce. Have one of them check the court order. I wold check with an attorney before you try taking the current deposit (I'm assuming she is not paying rent on the other place she rented from you) because it goes with the lease that they at one time shared.
Note this answer does not constitute legal advice, and should not be relied on. Each situation is fact specific and court specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship