We thought a riser because in order for her withers to be disappearing that way the front of the saddle must be lifting. We thought maybe if we lifted the back up by the same amount it would level it out and correct it. None of us has ever used a riser, so it was just a theory... which is why I posted for advice.
They're not going to buy a new saddle for a horse they're hoping will sell before Christmas, but a pad would be okay.
Thanks for the advice guys, keep the opinions coming!
Suzanne