Security deposit: Special roommate situation

My boyfriend lives in Boston. He moved into a duplex w/ 4 other roommates. Had a falling out w/ one of them. He moved in Sept 1st, moving out Dec 1st. He didn't sign a lease. Now the roommate is asking for him to give a security deposit so she can refund the former roommate his security deposit. Is this legitimate? -SZ

If he needed to give a deposit, why wasn't this required of him prior to move in? Normally, a deposit and first months rent (and sometimes last months rent) are taken prior to moving in. In a roommate situation, it is normal to request the deposit from the new tenant to repay the last roommate since the LL won't release the deposit he has until all roommates move out. However, it is done at move in, not 2 months later. And if he agrees to this, he needs to make sure a move in inspection is done so he won't be charged for damages made by the last tenant or before he moved in. Personally, I'd just refuse to pay a deposit now.