allow additional tenant to occupy under old lease – against my objection

I co-signed a lease for my daughter and granddaughter.  She got behind in the rent and the landlord, without my prior knowledge, allowed my daughter’s boyfriend to move in without my prior knowledge.  After I found this out, I told the landlord that the new tenant could not stay.  He told me that it was his decision and he was allowing the boyfriend to stay.  I believe that at this point, the lease which I co-signed was broken and a new lease was established between the boyfriend, my daughter and the landlord for which I am no longer liable.  None of the changes were ever put in writting, however the landlord did take the boyfriends personal information and tell him and my daughter that he was now on the lease.  This landlord does not do much in writing.  He does not even give rent receipts.  If anyone knows any maryland law that applies, I would be grateful.  Thanks.