If it is someone you know, is willing to help and you trust him then it probably is a good deal. Get a couple of things in writing to protect BOTH OF YOU and to not ruin a friendship.
Condition of use..... use for feed and keeping bull in good condition.
INJURIES ..... this is important. No one expects anything to happen.... BUT IF IT DOES.... spell it out who is responsible for what. Example.... we borrowed a bull from a farmer/breeder.... The bull did good then he "broke his penis".... we had established a value on the bull if he had been sold as a breeder.... he was one of 3 they didn't sell at their bull sale, decent but not top of the line. So he was worth 2000 as a breeder.... took him to the stockyard and they got the check and we paid the difference to the owner to equal the 2000. ONLY FAIR because although it was a "freak accident" it still happened on our watch. We were good, they were happy, and we used another bull the next year. This is when we didn't have many cows and did not want to buy and use a "stockyard" bull.....had done that and wanted better calves. 2 years later we bought our first bull at a bull sale. We have bought a few at dispersal sales too that were not always registered, and got a few good ones and a couple of so-so.....
Other than that, as far as injuries go, feet trouble and such is usually on the person using him..... hey, you would have to fix his feet if he was yours or he wouldn't be able to breed.....
One of our own bulls, turned up shooting blanks..... we had him for several years, used him and liked him.... never semen checked... and one year saw several coming into heat that should have been bred.... preg checked; open, open, open.... shipped him and put in 2 young bulls to catch them up... took 3 years of "moving the cows "back" 6 weeks or so, to get them back to where they should have been.... our "bad" . Sh$t happens.
Length of use so that no one feels stuck, or cheated out of time. We had one guy that wound up keeping a bull for over a year because he liked the calves and wanted to use him again.... we were going to use him that fall, but had other ones so we said okay. Got him back last year... fat and sassy....
Doesn't have to be a big deal, written contract.... just the basics so there are no mis-interpretations and will keep the friendship intact if something does happen......