She died about a month after this photo was taken and was weighed, ON A SCALE, not with a tape, in her necropsy at 1290... which would mean that following the 20% rule, she could carry up to 258lbs... which is equivalent to about 18 stone.
I AM getting at a point here, not just playing a guessing game.
You can't look at a person and or horse and make the judgement call one way or the other, whether the horse "appears" to be in distress or not. Some horses are much more stoic than others and if their rider surpassed the "20% rule" (which I kind of call bollocks on anyways, unless you have WEIGHED your horse with something besides a deceiving weight tape!). Someone might LOOK 250lbs and actually weigh less based on their body structure, or somebody might LOOK like they weigh 14 stone (again, thanks for the compliment) but actually weigh 250lbs. Then you look at the horse - I would never have guessed my mare to be 1290lbs, and to be honest, I am not sure what she taped at (but in my experience, they are not all that accurate). Without the numbers in hard, cold fact - can you really make the call fairly?
This is the same reason why while I don't have a problem with stables with rider weight limits, I DO have a problem when the way that they "weigh" the rider is by what their EYE tells them the rider weighs.
I really think you need to look at each horse and each rider individually with consideration for all factors beyond the weight of the horse and weight of the rider + tack. Experience, conformation (of both horse AND rider), fitness (of both horse AND rider), etc.