After a few more listening I amend my opinion on "Girlfriend." The main chorus material is the same, "Hey, hey, you, you," and then one follows with the word "boyfriend" and the other follows with "girlfriend." Musically, the "Hey, hey, you, you" is sung on one note and on the fifth of the chord.
The remainder of the two songs are different. I would submit, however, that the elements of the two songs that are the same, i.e. "Hey, hey, you, you," and "boyfriend," "girlfriend," are what make them identifiable and in the end, popular. So after more consideration, Lavigne did steal the song, or at least the "money" part of the song.