The landlord can hold any funds that you put down at the start of your lease and use them to cover any money you owe at the end of your lease. This includes amount of the rent for the time you were there. When two or more people sign a lease, unless the lease says otherwise, they are jointly severally liable for the entire amount of the rent, which means that if one person can't pay their share of the rent the LL can go after the other person for the entire amount of the rent. Once your lease has ended though (after the 30 days) you are out of the picture and if your ex-roommate can't cover her rent then the LL can only go after her for it. Hope that's helpful, good luck.