Dispute Between 3 Roommates and a Boyfriend

[John Casmon]

BP, I need your thoughts with an issue. 

In short, I have 3 female roommates where one has been there for 3 years (before I owned the place), one has been there for a little more than a year and the third just moved in a couple months ago when a new lease was signed. The tenant that has been there the longest has moved her BF in and upon finding out I confirmed with her that the roommates were fine with him living there, did a background check and pushed to add him to the lease. She did not want to add him to the lease and assumed responsibility for him.

I posted about it here - Tenants Bf has apparently moved in unannounced

There has been a rift between the roommates and now they can't co-exist. I want them to work it out amongst themselves and I'm cautious about saying something that may come back to bite me.  They're all exploring their options and have asked me if they can force the BF to move out. The new roommate claims she did not even know the BF would be living there. 

Can they mandate that the BF moves out? Should I? What are your thoughts?