Think about it this way. Especially with college loans, when you have a co-signer typically the loan is a good loan with the one person but they may have a little risk. Not in the ability to re-pay the loan but uncertainty that is created by the lack of history, employment or income. I know the name of the loan is a Parent loan in this case but clearly it is for her to go to college and her parents had no intention of paying 100% after graduation just by the fact that both names are on the loan. If they were going to pay the loan off, they didn't need her to co-sign for them because she would have added nothing to the application from a risk mitigation perspective. They co-signed for her loan.
Banks lend to your fiance because most kids go to college, graduate and get a good job where they can afford to pay back the loan. If they can't, they get married and jointly the 2 grads can pay off their loans or mom and dad help out. Additionally student loans are very hard to be discharged in bankruptcy and what new family/grad wants to start their career that way. From a lender's perspective the loan was to your fiance with a backstop of her parents if she defaults. Your fiance should be the one talking to the bank not her parents - it's her loan.
However, think about it from the bank's perspective:
1. It's you fiancee's loan. She is a recent grad with a reasonable income.
2. She has no other options by herself. She can't take on the unsecured loan by herself and has nothing to secure it with.
3. Mom and dad are probably not gonna use their House to secure the debt because that would be crazy.
4. Therefore, why would the bank reprice the debt from 7.5% to something lower.
Bottom line is you fiance's parents felt no obligation to help her with college and never manned up and told her that.