I’ve had an interesting experience with this.
My childhood instructor, from age 8-20, was very open about weight. She plainly told me I was too heavy for certain school horses. She was also very complimentary if my weight went down. She was very specific about the build of horse I needed, should I ever get my own horse.
The owner of a ranch camp where I worked in college, a British woman, was very conscious about the weight her horses carried. The 6’2" sturdily built man shared the same staff horses as me (I’m 5’5"); these horses were usually short, as we had to mount from the ground. Heavier campers were assigned horses first, since not every horse was up for carrying weight. The pony I rode when I was first a camper there was still carrying fat kids 12 years later when I worked there.
My current instructor is a bit different, she has told me “Stop saying you are too heavy! Any decently conformed, healthy horse can comfortably carry you!” I weigh 235 now, and purchased a Norwegian Fjord a few years ago to have a suitable mount. My instructor has encouraged me to get another horse, but the reason is to have a smoother, more cooperative partner. Friesians are bred at her farm, and if there were anything under 16hh I would consider it, but I really like shorter horses (14.2 - 15.2 is ideal). I think an old-style Morgan is likely in my future.
Regardless of the age of the rider, I think a straight-forward approach is the best. Instead of the suggestions to lose weight, I much prefer the approach of “Let’s talk about the kind of horse that’s the best match for you”, which opens the discussion of all equine qualities, including conformation and weight-carrying ability. For the woman with the beloved Arab, let her know that as the horse ages she will need to modify what she does with him to keep him comfortable. My Trakehner went from my full-time ride, to my part-time ride, to a ride for a lighter college student, to my bf’s occasional walk/trot ride in a western saddle, to pony rides, to full retirement. He’s still part of my life, but I haven’t been on him in over three years.
I’ve spent most of my life worrying about being too heavy for riding. I decided to buy a horse specifically built for carrying weight and haven’t looked back. The poster who mentioned changing her school horses to suit the heavier and taller riders she sees today is spot-on.