Helmet or not - hit a deer that jumps in front of you on a motorcycle going 60 - it’s gonna hurt.
Sucks - but there’s a reason why these things should not be on road.
Running a SxS with a cage around you is one thing - 4 wheeler with no protection is another. At least on a 2 wheeler you have some degree of maneuverability - 4 wheeler has all the steering ability of a damn golf cart but the horsepower of a large Harley.
Feel for the kid - quite the fall in future. And for his girlfriends family - imaging having to live through this going from a graduation party to a funeral in the matter of a week.