Personally - that sounds a lot to me. Depends entirely on the build of the horse though and the amount of bone it has. Could also depend on what the rider will be doing - a gentle hack may be fine but the horse may not be comfortable with anything more strenuous.
It depends on the breed and how much bone it has, my horse is 16.2hh but is slight so not really a weight carrier and would carry tops 12stone. It also depends on how the rider rides, in my opinion alot more heavier riders actually ride alot lighter and nicer than say your average size rider so taking that into acount and the build of the horse i dnt see it being a problem.
basically i have a neddie which had to have a year of because he got kicked above his knee... long story short my friend has helped me out by riding him but he's been very fizzy so thought if i ride him once a week just hacking it may help... he doesnt get any feed just grass.. he's 18 yrs old and it a french trotter... all i would do is walk him out hacking... will he be ok???
As others have said it depends how much bone he has. And also the density of that bone. I've seen 13hh ponies carrying grown men but they are natives bred to carry 20 stone stags off of the hills and moors. Then you see 16hh + horses with spindly legs who shouldn't be carrying anything heavy at all.
To be perfectly blunt, I wouldn't think a trotter would be bred for strength, so I'd say not in that case,especially an older one that has had leg issues. If it was a cob, then yes...x
No! And def not a lighter/aged/injured animal such as this one. Maybe consider getting a more experienced sharer for once a week (no money has to change hands or, if you're lucky, they might pay you/do some chores) or pay someone else to hack-school for a short while. Obviously if you'll not need the sharer long term it would be better to be up front/pay someone. Would certainly not risk 16st though.