I think it depends on the individul situation. I sent tack, grooming kit, rugs, his tendon boots, headcollar, leadrope, bsically what he would need and any extras she wanted to buy him she could. I also listed it all exactly inc description in the loan agreement. I used the BHS one. I would rather know he was happy and confortable and used to wearing stuff than them buying somehting he didnt need or didnt fit.
Everything. I sent my horse with all her tack, rugs, grooming kit etc, as I was having a complete break from horses whilst I went off to uni, so had nowhere else to keep it.
Make sure you do an inventory of everything though, to be included in your loan contract. You'll need this at the end of the loan to make sure you get all your stuff back, its also good proof of exactly what the horse is going with.
Mine went with saddle & bridle, and all rugs as they were all bought for him and fitted him well so I knew he would at least be comfortable. All items were listed in the contract as well as agreement to keep in clean and well maintained condition.
Have you found a loaner for F? I have the loan agreement I had when I loaned Ellie, complete with inventory. If you want to borrow it to copy let me know and I will pop it down.