I was going to mention Abuse also. I've never really played it much, but it's supposed to be "the" side scroller, and it is completely open source (the company went under and made it available).
If VGA is mostly just learning to make games, he's not going to be making any MMORPGs or clones of Grand Theft Auto. Let's be realistic.
A port of an existing game like Abuse, or a simple original game, is more feasible.
