What's the downside of marrying a friend to give them health benefits and greencard?

We would go to a lawyer first and get a prenuptial and anything else we can to separate finances. We probably would not live together and would divorce in 1-3 years, after they'd gotten medical treatment.

Edit: Which part of this is illegal? What if we lived together?

Edit 2: Oof, now I see how serious this is. I've scrapped this idea

Tc: 🥜