^^ In some towns there are unconstitutional laws that nobody has the funds to challenge that restrict unrelated felons living together. Usually while on parole. Point being to block halfway houses or sex predator group homes.
For the OP - give the tenant a notice that "it appears XX is living on the property. Due to being seen at various hours, at times when the tenant is not at home. Has been seen with keys? Car parked there overnight.
If this is a multi unit property install exterior security cameras showing who's coming and going. This one thing stopped virtually all our previous problems with unregistered occupants. Because while I can't see who goes into a specific unit, the tape will show who enters the common areas. There are a couple of units where someone could climb the back wall and climb in a back window, but nobody has gone to that extreme, yet.
They can't argue with a camera so our tenants now follow the rules. If they have a new BF/GF and the occasional overnighter turns into a "over-weeker" and they can't bear to be apart, the tenant let's me know and does the application process.
Get him out before he starts getting mail there!
I think I'd just tell here you will let her out of any lease and she needs to start packing.
Life's too short to deal with such nonsense.
--75.85.xxx.xxx