Nobody, I have close personal involvement with a few people who double dated and were on both sides of the coin.
While I understand the pain your child went through, rejection is a normal part of the shidduch process. Its a normal part of ANY dating process. There is always someone “better” out there and its just a matter of finding the right person for you. I’m trying to figure out a way to verbalize my thoughts well and I am having trouble at the moment. But your child should have understood that she was being rejected because the guy felt the match was not 100% compatible, whether or not he was dating anyone else. Double daters don’t sit there with a list and say “OK date with person A had these positives, person B had these positives. Lets compare the two and figure out which one I should dump.” Once there is some sort of committment (I think more than a few dates), then double dating is really cheating.
A close friend of mine broke up with a guy after dating for a while and dated up a storm for about two weeks (a lot of first dates and thats it). After those two weeks she realized what she had given up and called to see if the guy was willing to start up again. He was and they are happily married now. She needed a brief time to compare what her feelings/thoughts were before being able to move on. Does everyone need it? No, but some people do.