I have no doubt it was the reason, especially if she picked the wrong lender - one who lends to people who stiff amex at a higher rate than most people stiff amex. (which is probably every mortgage lender)
As for your reason #1 - looking at it from the other prospective, they might think she'll run up the amex then pay it off with the heloc and eventually run out of funds and BK amex. plus there is no guarantee she has a lower rate on it. #2 - not all lenders are cautious.
amex is running scared - new accounts, too many cards in use, along with the wrong mortgage lender are all CLD triggers.
My mortgage lenders aren't considered "wrong" lenders, however I do have a VERY high amount of available credit with my credit card companies.