There's a LOT of variables with tendon injuries. I am dealing with own, on one horse, and the outcome/treatsments are completely different from what anyone else has posted thus far.
And a HUGE DITTO on "Wallaby's" comment regarding hoof angle affecting stress (or lack of) on the legs.
To make an analogy, it would be like you having torn or stretched tendons and somebody expecting you to walk all day in a pair of 5" hooker heels
Hoof angle is very critical so I hope your vet has some leg & hoof knowledge and is willing to work with the farrier. There's not much margin for hoof trimming error in these situations. I would opt for barefoot and 4 - 5 week trims; barefoot would also save you money as opposed to shoes.
That all being said, you don't "sound" enthusiastic about paying board on this horse anymore. From the one side I can understand that. Is it possible the Seller will take the horse back? Were it mine, I sure would but if the Seller's in bad financial straits ORRRR knew this horse had a permanent injury to begin with, they may not.
The horse just can't be turned out 24/7 to fend for itself in a pasture full of other horses to save board money. Just imagine when a horse that is higher in the pecking order figures out yours is on the weak side due to an injury will do. Pretty soon you have a horse that is further injured and perhaps will never be useable for anything.
Believe me, my injured horse is permanently separated from my other three because Horse #3 is one Big SOB of a Bully and will Seize The Moment if he can. He is 18 yrs old and 16.1H; he tried to tear into Horse #2 who is only 13.3H and coming 27 years, the other day. Well guess who got the snot smacked out of him? It was not the 26 yr old but you get my point.
You are new to horses so that is why I am trying to explain how all this works when an injured horse is turned out with a bunch of healthy horses 24/7; especially when it's 20.
If you have the option of other boarding facilities, I would look into something without an arena and that has a run-in shed or a separate run-in stall to where the horse can exercise 24/7 and go outside when it wants to.
In the end, I can pretty much see how this is going. Very sadly my closing comment would be to try and sell the horse to someone who is an experienced horseman, has their own land, and is willing to take his sweet self as a pasture pet and maybe an occasional hack down the tractor lane
I hope this doesn't come across as ugly - that's the trouble with keyboards -- no voice inflection. I am just trying to point out reality