Well, I hope it wasn't very expensive fabric! And, I hope there was enough on the bolt so she could come back and buy more. At least she wasn't ordering fabric for a whole room of windows. If she was young...it must have been the first pair of curtains she was making. She may end up with cafe curtains! She will learn! So, don't feel bad.
However, IF I had noticed, I MIGHT have started a conversation and asked how deep the hems and header would be....at least I hope I would!