Letting someone else drive my rental car.

Hey guys. I'll be taking a cross country road trip in May and one of
my friends is under 25 will be tagging along for the ride. He won't be
doing much of the driving but in the event that I need for him to
drive for a bit, what are the risks involved with him driving but not
being on the rental agreement as an additional driver. I don't want to
add him as an additional driver because it costs way too much and he
won't be driving much.

If he's driving and we get pulled over for speeding, is it illegal for
him to not be on the rental agreement but be driving?

Also, God forbid he has an accident while driving, what're the
repercussions of that?

Thanks.

-ASR