I showed mine side saddle last year in good company at county shows and nobody said a dickie bird about his nose net!
I think if your friend just goes ahead and sees if anybody says anything then have a letter form the vet at the ready in her pocket just in case.
I'm sure nobody would actually bar her from the ring but if the judge is looking for an order for placings then they could argue that headshaking is an unsoundness.
I personally wouldn't put a nice horse down the line if it was wearing a net.