I have never done it, and I can't think of anyone I have known that did it. I have seen others loan a saddle to someone, or let them take a saddle that was for sale on trial.
If it was a saddle that I wanted to keep, not sell, I would be concerned that it would get damaged some how. If I were to charge enough as a lease to cover any potential damages (either as a deposit or as a monthly fee), I would think the person leasing would have more incentive to buy her own saddle rather than throw her money into a saddle that she wouldn't be able to keep at the end.
A friend of mind bought a brand new Lemke dressage saddle a couple of years ago. She had just untacked her horse and she sat the saddle on the rack next to the cross-ties. She walks into the tack room and comes out 5 minutes later and discovers that the BO had been walking a horse past the cross-tie area, had her cell ring and stopped to answer it. While she was occupied with the call, the horse she was leading reached over and started chewing on the Lemke.
If it was someone I knew and trusted, I'd just loan it while it wasn't being used. Maybe a contract stating that it be returned in the same conditioned it was loaned. And make note of any major blemishes on the saddle.
Equestrian Imports rents saddles out. Many years ago I rented one before buying, if I remember correctly it was about $175 a month. I'm guessing the new boarder wouldn't be wanting to pay much, if she can't invest in a saddle of her own. Personally, I wouldn't, if it was an old saddle I had no use for then I would sell it cheap but not lease it. Sadly people tend to not care for other person's property very well. If it's a saddle you love, pass on it. Direct her towards some good used or lower priced new saddles she can buy.
But I bet they require a substantial deposit on the sidesaddles and when I have taken a saddle on trial, the shops had a CC# should anything happen to the saddle.
Main concern here are the "what ifs" that routinely occur in a saddle's lifetime of use and hard knocks. Like damage from a horse dumping the rider and scraping the fence as they tear around, maybe hooking and iron on a post or standard ripping the leather off or going down on it into the jump poles or even just the ground and warping the tree. Or "ground damage" from getting knocked off a stand unto a concrete floor, dropped or something like that. Or even walks off, particularly if it goes to a horse show.
Would this person be willing to hold an estimated value of the saddle aside to cover mishaps that render it temporarily or permanently useless? I mean, you can't insure it and if something happens? You are out of luck with an "ooops, sorry".
I always assume the worst because I have seen it all and come to expect it as far as leases, trials and loaning things go. Many people, if it does not actually belong to them and they did not suffer to pay for it? They don't treat it so good, whatever it is. Because it's not theirs and is going back.
Don't even know what a used Passier would be worth to set any kind of value to figure a lease or rent fee would be. Even if it was, say, 1,000 that would break to 333 a year if you used the 1/3 stated value per year formula and, what, 29 or something a month? If it goes wrong, that's not much help replacing it and not much of an incentive for them to take extra special care of it. IF you trust this person, just loan it to them for a short term (like no more then 30 days) and help them locate a used saddle to buy. Or sell her that one. We want to be nice and help others but...this is a NEW boarder as well. I[d be careful.
When opportunity knocks it's wearing overalls and looks like work.
I lease out saddles - but not for a regular every day user. In your friend's case, she needs to buy a saddle. If yours is not for sale, she probably needs to buy a different one !
I do lease saddles, but it will only be where I believe the horse/rider needs a particular saddle only temporarily. For example, a newly OTTB with no topline at all goes to a new owner who has not trained a TB before. In that instance, the owner doesn't have a bunch of old narrow trapezius-paneled saddles hanging about and doesn't need to buy one because the horse is going to change so fast. Then I'll lend them something narrow with the right panel and the horse blasts right through it in three months and then we're in a position to get them a longer-term (1-2 year) saddle. And at that point they can buy.
The lease rate is about 7% - it's roughly the interest on the money I have tied up in that saddle.