I have a BMSS and LOVE it! I have the endurance model. It is SO comfortable and very secure, though I don't run barrels
. I do know some barrel racers who use Bob Marshalls and like them.
The Original Bob Marshalls are a bit better than the Circle Y versions. The one you posted looks like it's in good condition, though I would offer her $750 for it.
The barrel saddles tend to run a bit small, IF she measured it the correct Bob Marshall way (measuring the thigh opening, then adding 10"). If she measured it front to back (like a normal western saddle), it will run the same as your regular treed saddle.
Keep in mind that Bob Marshalls NEED a specialized saddle pad, to distribute weight and create spinal clearance. The best is the Skito treeless saddle pad
. For a low withered horse, I'd get the felt bottom material and grip strips on top, (regular, not dryback), and 1/2" foam with 1/2" lamination. That should make things nice and non-slip, and help so your hips aren't spread as wide.
I would also recommend using a 100% Mohair Roper style cinch. That will help stabilize the saddle as well, without causing any rubs.