You say you use plenty of lube so hopefully that's true. I don't know what's causing it, but I think cheerios gave good advice. Air circulation is key to a happy clean vag. Have her switch to light tampons instead of pads for a few months and wear looser breathable (like cotton, not silk) underwear. It could be too sweaty or too dehydrated. See if that changes anything.
A Theory: The lack of air because of the pad could create something similar to diaper rash... Here is a description of one kind of diaper rash that sounds similar to her situation: "- Redness and chafing where friction is greatest but not in the skin folds (chafing dermatitis)."
http://rxinsider.com/monographs/diaper_rash.htm Maybe if the gyno doesn't know a dermatologist will?
Causes of diaper rash: (emphasis mine)
Diaper rash is most commonly caused by the
constant wetting and drying of the skin along with repetitive exposure to a variety of irritating substances._ Once the naturally dry barrier of a baby’s skin has been irritated it becomes red, warm, rough and can lead to a variety of infections._ The most common irritants are urine, stool, bacteria from urine and stool, ammonia - which is formed by the baby’s stool breaking down the chemicals in urine,
detergents, perfumes, plastics and dyes from single use disposable diapers, pre-moistened baby wipes, soaps and reactions to creams._ The term “diaper rash” describes several different kinds of skin conditions in the diaper area._ Below is a list of the different types of diaper rash and what they usually look like.
http://www.thenewparentsguide.com/diaper-rash.htm