Yes. Tell her she can find another place with her boyfriend.
Supposing it is not a violation of the lease or of legal occupancy limits. She may argue she is not taking up any more space. But an extra roommate is a major imposition.
In the case that all the roommates agree to allow him to stay, the rent (and bills) should at least be split 5 ways.
In the case that the bills are covered by the landlord (you had better make sure the landlord consents to a new tenant in any case), it is the extra person using the bathroom, kitchen, extra noise, etc.
It is unfortunately an all-too common occurrence that roommates try to impose a boyfriend/girlfriend on the other tenants. This is totally unacceptable unless all parties agree, including the landlord.
In a worse case scenario, if they fight or argue it will effect the entire group. I recall reviewing crime statistics at Rutgers University housing and off-campus housing, and there were many domestic violence cases among roommate/housemates- but in every case, it was between those who were Romantically involved.