First of all, you need to prove that someone else lives in the house and shares living expenses. You’ll also need to prove it’s not her mother. I have no clue whether California has a common law marriage or how many years you have to live together to be considered married in common law, but you might look into it too see if it applies and has any effect on her support award.

Unfortunately, unless a judge deems your ex to be married under common law and decides that it’s the same as a legal marriage for the purpose of terminating spousal support I wouldn’t get your hopes up about having the support order overturned completely. You may be able to make a case for a lower amount, but that’s assuming you can prove she lives with a roommate under and arrangement where they share living expenses. Even the best lawyer can only present the facts. After that, all you can do is hope the judge is feeling generous that day.