After you get the bill, contact the hospital or the clinic and told them that your parent was visiting you and they had very limited income. usually the hospital will send you some sort of community medical help request application form so you can apply it for your parent. After they review the application, usually they will waive most of the hospiatl cost. Then you need to do the same thing to the doctors who treated your parent. You have no obligation to pay the bill if you do not want.
My aunt hurt her fingers when she visited us and was treated in the emergency room, the hospital and the doctor waived all the cost of about $1400.00. i also helped one illegal chinese immigrant who hurt his leg to get the cost waived from another hospital.
In general, it is a common practice for the hosiptal or clinics to waive the low income people's cost. What else can they do? That's one of the reason we get higher and higher medical insurance every year.