Sounds like he may be criminally trespassing, esp. if he was a guest that has overstayed his welcome and now refuses to leave. Not on lease, not invited, not paying rent (I assume) - an eviction could also be appropriate. If he's harassing or scaring her, an OOP or IAH is another way to go. If your mother-in-law is an elderly or vulnerable adult, and the BF is taking advantage of that situation, Adult Protective Services is another path.
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I would serve him with a notice to vacate and the file a complaint for forcible detainer (eviction). Be prepared to obtain an order of protection if the boyfriend gets hostile.
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