Hi Arwen. You are a good person for noticing his need of care and trying to get it arranged for him...some people would not pay attention to such things, and just ride him when convenient for them and go on about their business! :0(
Are you in a position where you would be able to (once his hoof issues are addressed, of course!) ride him every day, or at least get him out of his stall for a groom and some attention and then maybe a walk around the propery? Some free grazing time on lead? Something to get him out of his stall?
If so, perhaps the owner would be willing to do a sort of "free lease" situation; making you his primary caregiver and "exerciser", which would give him a much better life, including some much needed DAILY exercise, and be watched over for things like when his feet need to be done, etc...and she can continue to keep him where he is boarded (for the moment) since you don't really have the space to take him on.
At least if that were to happen, his life would improve. You could enjoy him, his owner could feel better knowing he was getting SOME of what he needs, and over time, perhaps a better situation could open up allowing him to have a far better option (a new boarding barn with turn-out pasture space available for him? Something?).
Just some thoughts! Hoping the best for him and you all!