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Your ex’s cohabitation will bar the court from awarding him alimony. It sounds like you have a good start to gathering the proof you will need to convince the judge of his cohabitation. You may want to consider hiring a private investigator to further document the current living arrangements moving forward.
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I believe the girl friend moving out would poor effort to defend against your claim of cohabitation and that the judge would see through the sham.
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I wish you all the best.