This would be an ex post facto change. The government wouldn't be retroactively making the lenders guilty, it'd be a change in regulations. If this change was unconstitutional then you'd have situations like a company with a contract to dump industrial waste into a river going "oh sorry, I made this contract based on the regulations back then, so it's unconstitutional for you to make me stop polluting"

If you had a contract that was contingent on your ability to dump your waste in the river, and then a law was passed forbidding the dumping of waste, you would of course have to stop polluting, but your contract would be null and void as well.

Non-bankruptable student loans aren't "regulations" they are specific contracts between students, lenders, and the US government.