I think that the process/ ceremony of buying rugs is fantastic. From the mint tea to the displaying of the carpets, and the out of this world salesmanship on display. For me, the experience itself is well worth any overpayment she might make (and she will probably over pay). The drama involved is great. She is invarioubly going to be:
a) Coming from the sellers favorite country.
b) making a purchase that is really for her children and grandchildren
c) ruining his business with such low offers
d) there will be whispers as to how she can only have the deal being offered.
I think its fabulous...if you're willing to play.
I do agree that she needs to do some studying beforehand though. She should know in particular how to distinguish hand made from machine made and wool from silk. Trying to ascertain if a carpet is antique is another story altogether.
She should also know a little about bartering. Guidebooks often give a rule of thumb as to how inflated original asking prices usually are.
She might also ask at her hotel for a reputable dealer. I think walking into the Grand Bazaar in Istanbul might be otherwise overwhelming.
*As an aside, I think carpet salesman are all math geniuses on the side. THe ability to know, and perform all the currency conversions that they seem to be able to instantly do in just about every currency you can think of, is truly impressive.