Unless his probation would somehow prohibit him from being there, there's not much cheap or easy to be done.
Breaking the lease could be expensive, but might be possible if someone else were found to be a replacement. Depends on how willing the landlord might be to play ball. Now that they know WHY your daughter is wanting out however, they aren't going to want to play this game every month once the next new roommate finds out about this guy, complains, wants to move out, etc. - so, it's probably in the landlord's best interest to try to hold her there under the terms of her lease rather than start the revolving roommate routine.
Your concern is understandable, but actually, at least ONE college girl (the NEW roommate) wants him there. And unfortunately, parents have no way to control who a landlord chooses to lease to as far as roommates go, short of helping her to get a place of her own where who comes and goes is within her, and only her, control.not the type of person college girls or their parents want at the apartment
Depending on how much time is left on the lease, she could pay off the rest of the lease, or, if she's able to find someone to replace her, perhaps she could lease in a new unit (either by herself or with roommates of her/your own choosing). If the landlord doesn't want to be bothered by the issue, he might be MORE willing to cooperate if there's the possibility that he'd be leasing TWO units as opposed to one for his trouble.
It also couldn't hurt to run the lease past an attorney to see what restrictions on visitorsmight be contained within the lease, and what remedies might be available.

