My advice is to have your friend consult a Attorney.
I did when I had a track on my Farm, The Attorney advised me not to have anyone ride at all.
But he knew I was going to have a track and ride anyway, so do Not have anyone sign a Waiver. ( Individuals can not sign your rights away )
Note: A waiver insinuates that there is danger in riding on the property.
Yes, everyone has you sign a waiver, but they have insurance at all race tracks.
We road for a couple of years and had a person with 2 broken arms ( he had insurance and a office job, missed very little work)
Teenager Twisted knee, Father was a Doctor, had insurance, no problem
I am sure of many other bumps and bruises, no problems.
Remember in a Catastrophic injury, I am sure the ambulance chasers will file lawsuits.
Just my thoughts
Terry