Co-tenant moving out i'm staying, what are my rights?

[Amelia]

A formerly good fried of mine and I signed a lease for a three bedroom apartment in the fall of last year. She has recently lost her job and less than two weeks ago told me and our third roommate (not on the lease) that she is moving out. At first she was level-headed and considerate saying that she would find a replacement to take over her room and rent. Initially without much thought I didn't protest to her getting off the lease and having our third roommate sign on before we foud someone to rent the room, but our third roommate is a student and neither of us can afford to pay the one who is leaving's rent if we don't find a new roommate by the first (in three days). I finally woke up and called the landlord to ask him to please not let her get off the lease until we find someone to replace her. She was upset and has begun to act irrationally. She posted an ad on craigslist but respoded to people that we interviewed as a group without letting us know and essentially scared them away. She told two other potential roommates that we had rented the room when we had only offered it to someone who declined, once again scaring them away. She has left her empty room a mess and when I swept and through some trash away she wrote me another angry email about that. Basically, we are scared to let her play a role in searching for a roommate (that we will have to live with - and do not want to go through this again with!). So far she has said that if we don't find someone she will pay March, but given her flakiness I don't trust her to do that.

Now to really complicate matters, her mother paid our entire deposit (one months rent). Given that she is breaking her lease by leaving am I right to assume that we don't owe her that money. She mentioned in an angry email that her sister's roommates are lawyers and I need to know what my rights are. What can I do to protect myself? I've heard that all people on a lease are responsible for the entire rent, not just their share, but I don't have it! If she decides to pick up and leave without paying rent what happens to me? Does she lose her deposit?

Thanks for any advice and knowledge - I need it.