I would have said something. I have always been that way. But years of being scolded by older sisters who insisted I have no tact, I have learned to say or ask if I may be able to offer some advise. In this case..."I'm sorry I don't mean to intrude but you are going to need more of this "beautiful" fabric to complete your curtain." and then point out that she needs more for headers and hems. Notice the ego stroking.(goes against the grain with me, but it works) More often than not, people are receptive. If it is your nature to be helpful, God bless you and I hope our paths cross someday.