hmm, i dunno. it's a tough call, but if u tell her its fake, you may be saving her from embarrassment further down the line.
case in point: a friend of my mom's recently had an "LV" bag that had gotten scuffed/worn somehow (i don't know the details), and so she took it to an LV boutique for repairs. it was really a fake, but she didn't know it, as her daughter had given it to her for xmas and she just assumed it was real. well when she took it in to LV, the SA basically had to tell her (in front of everyone) that it was a fake bag.
she was beyond mortified. when i heard that story, i felt so bad for her. others here have mentioned how they seen people take fake bags into boutiques, and how humiliated they are when they're told its a fake. if u think it about it, wouldn't it be better if she was told privately by a friend than to potentially be "outed" in front of people?