I would say 60 lbs., tops, and that depends on the build of the pony.
Please make sure, too, that the rider isn't just plain too big for the pony. Golly, there's nothing that I dislike more than seeing a pony being ridden by a child whose feet hang to the pony's knees, or lower! It just looks bad. It's frowned upon when it's a "Mini" but seems to be widely accepted when it's a 'pony' Honestly, I don't see the difference. If the rider's feet hang closer to the ground than to the pony's girth, the rider is too big for that pony & shouldn't be riding it.