[RT Wolf]

Well, morally it's a tough cookie. I mean, on the one hand, B made a commitment. On the other hand, they provided ample (6 weeks!) notice that they're changing their end and helped to find several (!) subletters. In my book I'd let B off the hook and get to work on figuring out how to either get the roommates to agree to some subletter or finding a more suitable subletter. Are the roommates being nitpicky or do they have valid concerns? How can A address them? A might have to accept a slightly lower rent than they pay and cover the difference but I'd imagine that's preferable to having to pay the whole rent and not use it.

Hmm, if B is an exceptionally high character person, they may still offer to pay for the rent, but I wouldn't say they have a responsibility to and I wouldn't force them to using legality. I may ask them if they want to pitch in if no one can be found but not pressure them.

Legally, check with local laws on verbal contracts. I personally dunno if there's any way to check without having to pay for a lawyer but ask around your social circle, someone may know how to check local laws or ask specific questions about verbal contracts. Someone else here may be able to offer information on verbal contracts, I'm not a lawyer.