Easy mistake. It was an older bill with Monticello on the reverse.
Newer ones(twos?) have a portrait of the signing of the Declaration of Independence would have been easily recognized.
(/s)
I've never looked it up before, and all my $2 bills were of the 1976-present variety, so I'm only familiar with the reverse image showing the signing of the Declaration of Independence. However, I would have done a quick search on the older bill (as shown by the issue date of 06/09/62, and the series 1952 A), and immediately given the girl a fiver for it. I love the modern $2 bill for the image on the reverse side, but the one this girl had is fairly rare, and worth the extra $3.00 to me. Apparently it is to other collectors, as well.