I own a home in a neighboring Town that is being occupied without my permission?

We have an acquaintance that wanted to buy the house last year. He has a key and is paying our mortgage payment directly to the bank every month until he can find financing to pay me off. He doesn't live there but had a key so he could go in the house, make minor repairs etc and keep an eye one it. I just found out he has rented our house out to a third party without our permission. So now I have this Woman living there without my permission. (he told her that he was the legal owner). He doesn't even have a valid purchase agreement with me anymore. It expired. I just trusted him and gave him more time until I took the key back. He said he hasn't accepted any money nor signed any type of lease. How can I get her out? A termination notice is for tenants, right? She believes she has permission. Is it called unlawful occupation? Thanks for all advice!