To me, the answer really depends on the level of friendship. For example, my sister is, next to my husband, my best friend. If she gifted me a fake, she would not be offended at all by my letting her know as she would feel angry that she was sold a fake and would never intentionally purchase a fake. Telling the truth in love is the right thing to do. Not wearing the bag is also the right thing to do.
Also, any friend of mine would know enough simply from our conversations about LV to know where and where not to purchase such items and that counterfeits are rampant. She would most certainly visit an LV store or the official website.
ETA: however if this were a lukewarm friend or acquaintance, my answer would differ. I would not say anything but also not keep the bag. She wouldnt notice anyway.