If they have accidental health insurance coverage, why wouldn't their insurance pay? Wouldn't they pay 9 times out of 10? What if he tells his insurance he got hurt playing football in someone's back yard. I'm sure that happens fairly often and I am guessing the insurance covers it. People get stitches, break their bones etc. all the time....and insurance covers it.
First of all, he'd be lying.
Second of all, the point isn't whether or not his insurance would pay for his initial injuries. The point is, what if his injuries are SO severe that his insurance pays out and he's still not "made whole"? Or he reaches his cap?
What if he dies from his injuries?
What if he lies to YOU and says he has health insurance and doesn't? Then your homeowner's insurance will be expected to cover his potentially life long injuries/treatment/therapy.
There are a trillion what ifs involved. And at the end of the day, you're looking for protection from a lawsuit - and there is NO protection against being sued, only defenses. OS told you already, but you need to understand - even if you win a lawsuit, it will cost you. Perhaps 'just' a friendship, perhaps a lot of money, perhaps everything.
If someone comes into my yard without permission, and tries to break into my house and my mastiff bites them (or my beagle, or my collie for that matter) and they're injured, they can sue me. Do I have a defense? Of course. Several defenses. But it's going to cost me to present them, and I may not win my case.
You need to do a little more cost/benefit analysis and a little less attempting to CYA.