My understanding is those agreements are unenforcable because your friend is being told to sign it under financial duress ("we won't give you any severance unless you sign this" is duress). He should talk to lawyer and not listen to some anonymous person on the web, but if it were me, I'd sign it, get the severance, and then think about suing anyway.
My only other advice is to get the COBRA or find some sort of health insurance, even though it is expensive and may seem unneeded. Then find some job, any job, that includes less-expensive health benefits to carry him through for a while. My best friend was laid off about six months ago, and in the last two months has started having health problems, namely, kidney stones from hell. A couple of trips to the emergency room, some doctor visits, some prescriptions, and the imaging used to look at the stones altogether is up to a total of $13,000, and without insurance or an income, that hurts a lot. He's in his late twenties -- you wouldn't expect health problems at that age necessarily. At 50+, it's going to be a priority to keep health insurance.
Final piece of advice: maybe this is his opportunity to go into business for himself. After all, he's not likely to lay himself off
