You can't 'give 6 months notice' that isn't how it works. You give notice. The length of it is stated in the contract.
She is no doubt putting their needs first, in which case the best thing to do might be to have a chat with them, let them know she is looking to leave, and ask how long they would want her for and go from there. If they say 6 months, she still doesn't give 6 months' notice, she still gives whatever is in her contract. They then ask to negotiate a finishing date, and she accepts it.