Unwelcome guest in basement of rental

[Stephen C Gates]

Recently purchased a fully-rented four unit apartment building. Each unit has a storage room in the basement for tenant use. Last week I discovered one storage room being used as a bedroom by a friend of the tenant. I spoke to the tenant who said it was just that one night, she didn’t want him to drive drunk, but I’m not buying it. He has shoes neatly lined up in there and personal property, etc. I emailed her a copy of her lease highlighting guest stays and no subletting or license policy and told her use of the storage room as guest quarters or living space is grounds for termination. This was last week. I was at the unit doing maintenance today and I saw the guy there, he looked like he was going down to the basement until he saw me and went back into her apartment.

How do I go about discouraging this behavior and getting this guy to move on without having to evict the tenant? She’s actually paying the highest rent in the building and has a newborn baby, so I’m loathe to go through the whole thing so soon. Can I just call the cops and say he’s trespassing? Should I install a security camera for proof that he’s staying there? I’ll evict if I have to but I’d rather work around it.