"If friend had checked the oil level before firing it up", he would have seen an Overfull oil level condition, & possibly even smelled the gas now in the crankcase.
Anyhow, I had a 2008 ylt3000 that did same. I put in a shut off as well, after it had filled the crankcase on that one I used to have.
If it is a stick (6 speed) & is a Dana transaxle, that is gonna give you problems & there are no replacement parts.
But otherwise I'd say it should be worth $400. I'd try & haggle for lower price tho.