Old loner won't stop pestering me D: what should I do?

If she hasnt already clued on to the facts then you may need to spell them out to her unfortunately. I think its rude that she would turn him down then get another horse and just because its lame she wants him back. 6 days a week riding is enough for any horse so I'd just tell her your sorry, its not viable as your horse is in work enough as it is. The cheek of some people :) Bet you'll never remain friends on facebook with any future past owners!