yes, it does affect your credit if u do not pay. Its good that your friend is making monthly payment and 100$ each month is not big amount compared to $45000. If your friend stop making payment then all the bills will be send to third party(collection agency) and your friend will be liable to pay all the amount left in the limited amount of time unless there is some agreement between your friend and hospital. Health insurance is must in United States otherwise you will be in great trouble financially if you ever have to be admitted to hospital.