Back when the Hurt report first came out, I remember being impressed by a statistic that said most accidents happened to new riders (first 90 days I think) or those on borrowed bikes. Riding an unfamiliar bike can be problematic if you encounter a situation that requires intense braking or maneuvers.
As for loaning a bike, you really want to know something about the rider's skill and mindset. Even a minor tip over on a faired bike can be really expensive. I've also met a few would be test pilots who gave every vehicle they borrowed a 10/10ths workout.