It might depend on the hospital - I don't think (and I'm no expert) a county-funded hospital could refuse treatment on the basis of not being able to pay. Especially for rabies, which would probably kill him if he had it.
I think that might depend upon the state. In Illinois, they have no problem turning you down unless you are signed up with Medicaid in advance and have the card with you. (In my area, a single person has to make less than $16,000 per year to qualify for Medicaid.)
Here it's what J's Annie said -- pay in advance or hit the door.(Remember the news story in Sept. 2007 about the woman who literally died in the waiting room of a California hospital? Her husband dialed 911, but they wouldn't send anyone, because she was already in the emergency room. The hospital wouldn't treat her because she was uninsured and the staff had recently been warned about accepting too many non-paying patients. Nurses literally stepped over her as she was convulsing on the floor.)
Most hospitals and doctors will work out a payment plan for the copays if you are insured. (After all, they are still getting the majority of the fee, from the insurance company.) That doesn't mean they will do anything similar for someone who is uninsured. Most hospitals are eager to keep debt/loss to a minimum, and they do that by not accepting patients who can't pay.
I'd have a lot of trouble believing that the staff of the hospital would intentionally commit insurance fraud on this guy's behalf. For one thing, if they are caught, every staff member who was aware of the problem could go to jail and/or lose their license to practice. For another thing, the entire hospital might lose it's accrediation over the issue. It's just too big a risk to take.
And, why is this guy so special to the hospital staff? Trust me, they see -- and turn away -- uninsured patients every single day. There are millions of uninsured people in this country. Doctors and hospital administrators can't afford to feel sorry for every one of them.
As a freelance writer, I don't have insurance. I've had the same doctor for 12 years. He charges me $110 per office visit, instead of $120 -- that's the extent of the "empathy" extended to most patients without insurance. And, it's cash on delivery, no payment plan. (And he's a nice guy!)
I would think a much more likely scenario would be that your character would use his friend's insurance card, without the hospital employees being aware of it. So Joe Blow would pretend that he is Steve Smith, to use Steve Smith's insurance card. I've noticed that doctors and hospitals seldom ask you for i.d. in addition to the insurance card. (Maybe if they do, the two look enough alike that Joe could use Steve's?)
It could start as a mix-up and just progress from there.