What do I do if my ex lives with a partner but will not get married because her lifetime alimony ends?

In order to accurately answer this question, I would need to read the terms of your specific settlement agreement as that document would explain what the triggering events would be to modify or terminate your alimony obligation. However, if your agreement indicates that cohabitation is a triggering event, then you would have to prove that she lives with this person and that this individual is contributing financially to her needs to the degree that her alimony needs are reduced. This would all have to be done via filing a motion with the court and many times, a private investigator is utilized to try to prove the cohabitation. Typically, this is not an easy motion to succeed on as it can be difficult to prove the financial contribution by this individual to your ex spouse's needs, but I hope this helps answer your question.