Your friend should contact a lawyer who knows the laws better than random strangers on the internet.

If the entity still exists, sure. If it does not, then there is no one really to sue.

You can always sue up until the statute of limitations for that particular tort has expired. Just because a business has ceased to operate does not mean they can't be sued. It just means there is a greater chance that the business has filed for bankruptcy and there is a lesser chance of getting anything if you win the lawsuit.

That's a "talk to a lawyer" question. 

I'm sure the hospital had insurance that would take care of it.

U can if they made u constipated

You can try, but if the hospital closed because it was losing too much money, his chance of getting anything is rather remote

First of all, what he believes is irrelevant. To sue a hospital or doctor, he must find other doctors willing to testify under oath that his mother's providers acted in a negligent manner or outside of normal care. It is very tough to find doctors willing to testify against their peers.

As far as suing, he could sue the specific doctor(s) that acted negligently. If the hospital is part of a larger organization that still exists, he could sue.

All of these are great questions for him to ask an attorney.