This is all I can think of at the minute but I hope they're of use!
Why is he being loaned out for just one year? (because I would be wary of loaning a greenbacked youngster for me to put some miles on the clock before he goes back to the owner to be either kept or sold on. Though I have done that in the past and ended up buying the horse I was putting the mileage on!)
How long has the current owner had him and when was he backed?
Has he done any competing? If so, what?
Is he good to box/catch/clip/shoe.
Is he good to hack on his own/in company (first and last) and what is he like in traffic.
Does he have any vices? Weave/crib/box walk.
Has he ever been known to bite/kick/bolt/rear/buck/nap.
If he doesn't like something when out hacking, what does he do? Does he spin round and try to head for home, reverse, stop, shoot past, or what?
Does he have a passport and are his vaccinations (flu & tetanus) up to date?
Is he insured? If so, will the insurance be continued whilst you have him on loan or will you need to insure him?
Has he had any illness/lameness episodes. If so, what?
If his owner wants him back, what is the notice period and ditto for if you want to return him?
What is he like in the field in company (I presume you're keeping him on a livery yard?) and is he good to turn out/bring in and handle in the stable?
Is his worming up to date? What has she been using and when was he last wormed (should have been an Equest in December).