The fact that you all sigbed the lease does not necessarily mean she is breaching a contract with you. You probably want to get any attorney to review the lease. Bit may give you veto power over the new tenant, but it may not. Run ads on your own for a roommate so you have some options.
This is not legal advice. Even if it were, fee legal advice is worth what you pay for it. The facts of every case are different and should be addressed by a competent attorney who has all of the facts.