Did you do an inventory of your motives? You sound super angry and maybe you were projecting your issues onto her. If he is in rehab and not driving her car, it's really her business who she chooses to live with.
You said he is "violent," but didn't give any details or evidence that the kids are in danger (and of course, if the kids are in danger, CPS should be called, but otherwise, you might not like the situation, but it is her business).
Just my two cents.