The terms of the assumption of mortgage will set forth your father-in-law's remedies. He needs to contact a real estate attorney as soon as possible.
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Your father-in-law, as owner of the property, needs a lawyer. It is not clear to me whether he owes the mortgage or owns the mortgage. It is unstated whether there is a lease. Those documents should be reviewed by the lawyer in determining how to conduct the eviction. An eviction may be obtained either by the occupier voluntarily leaving the property or by a detainer lawsuit in the county in which the property is located.
These comments do not constitute legal advice. They are general comments on the circumstances presented, and may not be applicable to your situation. For legal advice on which you may rely consult your own lawyer.