Tenant Boyfriend Practically Moved In

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What is the name of your state? New Jersey

I own a 2-family house in New Jersey. I live on the first floor apartment and have a tenant on the second floor.
Tenant is great, single woman, professional, always pays on time... Keeps the apartment clean. Have been living in there for 1.5 years.
We share our basement where she have a separate room for storage. I have my things in the basement too (which she have access to).
Recently she got involved with a man that is "visiting" her more and more often. He practically moved in about 2 weeks ago (just hasn’t moved his things). He sleeps in the apartment (with her) every night. Leaves in the morning to work. The problem is, I have no idea who this guy is. He is not on the lease...

What is the best way to handle this situation?

Thanks
YG