As Evoex stated YOUR insurer will pay to repair your bike and then go after your buddy for their costs. If it is your buddy's fault, (say single vehicle collision), the YOU would be out of pocket for the deductible. If the other person is deemed at fault, (your buddy gets rear ended, NO deductible is payable.
In first situation, (your buddy is at fault), YOU, Should you be looking for a new insurer, be rated with an at fault CLAIM.
Worse case scenario, your buddy loses control, mounts the curb and hits a gaggle of people waiting for a bus. You carry $2 million in liability, BUT the claims total $10 million... guess who is on the hook for the remaining $8 million. No one EVER lends out their vehicle thinking my buddy is a dick head, and rides/drives like one, but I'll give him the keys anyway, afterall, he WILL be careful with MY vehicle...lol.
Now in your scenario, of riding an uninsured bike, (even if for the purposes of helping a buddy out). You were to crash it. Your insurer isn't going to pay your buddy, for his bike, nor will they cover, ANY medical claims by you or anyone you hit. Then of course the ramifications of the law also kick in. It is YOU who would catch the operate motor vehicle no valid insurance. Should your insurer dicover that conviction you get dropped by them and it ios off to the facility market you go.
Could you elaborate on that? Let's say my buddy borrows my bike and crashes it due to his fault. I guess my insurance does not pay me since I was not the rider? What if he crashes due to somebody else' negligence? Still insurance does not pay me because I was not riding it?