Evict non-paying roommate?

[MK Wang]

Hi BP,

So here is the deal. My tenant allowed her exboyfriend to stay with her because the guy was laid-off and had no other place to go. But now the ex-bf is refusing to leave and not paying anything. She called cops on him but they can't make him leave because he is technically a resident of the house since he has stayed for so long. 

What should I do as a landlord to sort this out? Evicting my tenant would be my last option because she is never late on her rent and keeps the property in good shape.