Therefore, you can take the buyer and negotiate a fair commission with the agent. That would be the best solution. She is not entitled to the full commission.
Look at it this way...at least you have someone who wants to buy the property!
Refer to the original listing agreement to see if there is a clause in there about the brokerage being entitled to a commission earned if any prospect introduced to the property during the term of the listing purchases the property within 6 months after the expiration of the listing agreement. If there is such a clause, the agent should have given your fiance a written notice of any prospects. In this case, it sounds like this buyer was introduced to the property after the listing expired and the agent should have had your fiance sign a separate agreement pertaining to this individual buyer before introducing them to the property since she didn't relist the property first.