Regardless I think it is best to have a contract/agreement of some sort going into this. The quickest way to end a friendship is to go into a situation like this where you are allowing someone to borrow something, or into a partnership and not have a written agreement detailing everything. That way both parties know what is expected and at the end there cant be any hard feelings because everyone knew up front how it was going to work. I would say the biggest things in there should be a salvage value if something were to happen. That salvage value should be equal to the amount it would cost to replace the bull with a bull of the same quality if something should happen to him and he were no longer suitable for breeding. You should also have something in there about the condition you expect the bull to return in, how you expect him to be fed, etc. Also it should state who is responsible for the care, feed bills, and any vet bills incurred while bull is there. You would be surprised how many people "borrow" animals in this situation then either expect you to cover the costs or later on want to hold it over your head or say you owe them because they paid his feed bill for so many months, etc.
The insurance thing is a good idea, possible concerns there, if you add him to your farm policy, it may not cover if he is not on your property and not under your care. Also you may need more than just accidental death, if you are going to go the insurance route, you also need to have something in place to protect you in the event he is no longer suitable for breeding, there are many things that could happen and result in him not being able to breed cows, but wouldnt necessarily kill him. Some farm insurance companies understand that and if you put him down in that situation my honor an accidental death policy, but many are only going to honor it if the actual accident is what kills him.
But even with insurance I think you need to sign an agreement, not because you doubt your friend, but to preserve the friendship, that way everybody knows up front what is expected.