did she sign the same lease as you?
If so, does the lease allow you to break the lease with a 30 day notice?
If not, tell her her responsibility to pay for the lease is for the entire term of the lease. She doesn't get to skate out on her liabilities by writing a note saying she is leaving
If there is a right to terminate the lease with a 30 day notice, then I suggest you figure out what you are going to do in 30 days when THE lease ends.

