K.P.
Considering the fact that at 33 Santa still brings me gifts and the Easter Bunny still sends me a basket of treats in the spring... my guess is that if I were to lose a tooth there'd be $5 in an evelop for me too!
Play along. It may seem wierd to you because you're an adult, but when she loses the tooth and her friends ask what the Tooth Fairy left her, what will she say? "There's no such thing"- bummer for her and the other kid.
I hope the Bunny will still be visiting her this week- that would be pretty sad too!