People who run ATV's/UTV's can purchase insurance, very much like auto insurance.

Here's a link to one company:

http://markelinsuresfun.com/utv-atv-insurance/utv-atv-insurance-rates-coverage

If, and I say, IF you are going to allow this guy to do this, I would encourage you to have HIM have insurance. You can check out that page, and decide how much you want him to carry.

However, like the others, I question why..... why do you want to open yourself up to this liability?  What's in it for you? how will you benefit?

edit: I believe requiring HIM to get insurance does not limit your liability. However, I think you would still be wise to require it.