A casual friend asked you to be in her wedding? Kind of strange, but not unheard of I guess. Just a thought... is it possible that she doesn't have a lot of girlfriends and needs more ladies on her side? It's also possible that she admires your commitment to your marriage and family and would like someone who understands the whole thing standing up with her.
Regardless... did you already say yes? If so, you are kind of stuck. I was in a wedding in June for a childhood friend and her FG was 4. It was a LONG day and she was an angel... right until we got to the alter. She had a complete meltdown. Her mother was the MOH and couldn't just swoop her up, so I did. I literally held her through the whole ceremony and she fell asleep on my shoulder. Evening weddings and young children don't mix!
IF she wants your daughter in the wedding, make arrangements for someone to drop your little one off at the ceremony site just before it starts. She should be fine for the ceremony, pictures and an intro at the reception but a sitter should be ready to take her back to the house for dinner and bedtime!