Snowfox, you might be interested to check Dr. Deb Bennett’s articles on conformation and what horses can or cannot carry. You can direct your student there, too, as a part of her equestrian education.
She had a great series in Equus a couple of years ago (2009), but I think you can find it online, too. She has a webpage.
She seems rather abrasive, but her work on equine theory is very good.
Btw., large horses, such as drafts, being suitable for heavy riders is a myth. They are hardly suitable for riding at all. A pony with a stocky build would fare better.
I was taught by old cavalry guys in Europe (my first teachers)- you want to ride, you keep trim and fit! I know, in US, people see it differently.