Invite her to this forum and let her see that it is possible!
I too have bulldog type QH's and they can certainly handle a heavier rider. My hubby is 220 lbs of muscle, he rides several of mine with no problems. One of his favorites is our 14.1 cutting bred stud, he weighs around 980 lbs and can still move like a cat with my hubby on board.
I'm not plus size now but have been so I can relate. I never stopped riding but when I was at my heaviest, I didn't feel as 'in charge'. The only way to overcome those feelings is to ride. Start in the arena and take it nice and slow to build her confidence back up. I'm sure like most that have been out of the saddle for awhile and come back to it, she will realize how much she missed it and forget about her worries.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.