It depends. It would be worth comparing the interest being charged on those student loans vs. the potential ROI you could get from investing the same amount. If she has 0% loans or loans at a very low rate, then it might make more sense to invest instead. However, (I'll just assume for this that you are married and treat her debt as truly yours, and therefore sidestep the ethical/'should I?' questions) it's likely she has loans at interest rates of 6.8% (standard gov't loan interest AFAIK) or higher (if private) in which case you wouldn't be able to earn the money back via investment, and you should save yourself on all that interest by paying off immediately with the $100k. I feel like the "standard" ROI that a person is told they can "expect" is around 6%, maybe up to 8%. I have seen wider-ranging numbers but I think anything above that gets to the "counting your chickens way before they have hatched and maybe before they have laid eggs" point of reality.

I like reading personal finance advice and stuff. I'm mostly crap at managing my money but I still find it all interesting! :)