I'm so glad that she is going to join and read, there are lots of pics that us big girls have shared of ourselves riding, I hope they reassure her that she can give it a go.
Just to add some thoughts, and maybe some of the others will back me up here...
As a big person you just know that when you put your foot in the stirrup you will either
A) make the horse fall over
B) Pull the saddle right round under the horses stomach.
While A is unlikely, B remains a possibility, especially if you like me ride horses that weren't blessed with withers
To assist with the mounting it is beneficial to both horse and rider to have a good solid block to mount from, no not an upturned bucket, but something stable that we know will give us a firm platform.
You should ask her if she is more comfortable having one or two people around the first time she mounts, I had my son holding the horse, and hubby holding the off stirrup and being ready to support me.
If she used to ride at a lighter weight she needs to be prepared for the fact it will feel different, there is a moment when you feel that this is a scary and foreign place to be, but then everything kind of falls into place again.
I was going to say that any of the plus sized crowd would be only to happy to chat by pm if she doesn't feel comfortable sharing on the main board, but I think you have to have a minimum number of posts before the pm thing will work.