Make the claim to his auto insurance. Also, make a police report of an injury accident to further substantiate your claim, otherwise it will turn into a he said, she said scenario. In my state there is (was) a 10 day window for reporting injury accidents, so if you go this route make the claim asap. The driver may also be held criminally responsible for failure to report an injury accident, but my state doesn't prosecute this although they can, but many other states do.
If the driver is uncooperative definitely make the police report. You may have to pay the medical yourself and attempt to recover medical expenses from your friend in court.
My kid was in the same boat as you years ago, and wouldn't go against his 'friend'. Needless to say, I ate it for $1100 cuz the kid and his dumb momma wouldn't go against the friend, who btw is no longer a friend.
Now if you are going to marry this friend, avoid the police part if you can, but definitely get the insurance to pay the medical costs.