I am also looking into purchasing an existing massage business. It is a very different scenario from what you are contemplating, but the fundamentals aren't so different.
If she wants to sell you the business, she needs to provide a LOT of info to you (full disclosure!) before you sign on a dotted line. For that matter, I'd ask her to provide that info before you give her a counter-offer.
There are excellent references you may consult on starting/buying a business (that's what you are doing) and it may be helpful to educate yourself on the buying process. If you understand it better than she does, you'll have a significant advaneforum.xxxe.
Make no mistake, this is about you getting the best return on your investment. This is NOT about you giving her a "fair deal," whatever that means. You don't have to be heartless, but you should definitely BE A SMART, INFORMED POTENTIAL BUYER. This includes writing a business plan and having a small business lawyer/accountant review everything before closing a deal. You might realize it's a lousy deal and you're better off without it.
The formulas and such discussed previously in this thread may not apply to this deal. They certainly have no application to what I'm contemplating. You need more info before you'll know if they apply to your situation. Running a business entails thinking like a businessperson, or it won't work. You can be a great, caring MT and also a shrewd businessperson. It's a powerful combo, but rare.
Good luck!