If your tenant signed a fixed-term lease (i.e. a 12-month lease), and wants out of her lease, you do not have to release her. Since she signed a lease, she must pay you for the term of the contract. However, if she moves out you are obligated to find someone else to lease the property. This is called damage mitigation, and it is required by statute.
If she moves out and does not pay you, then you will need to hire a lawyer to secure a judgment for you. Usually, though, the best way to deal with these situations is to negotiate a resolution. Contact an attorney to determine your options.
I agree. With six months left, she has no right to move out (at least on the grounds that she is buying a house). If you would like to discuss with me further, I welcome you to give me a call.