If your idea was so simple that a friend could steal AND execute as YOU envisioned, your friend isn't he problem--your idea is.
MySpace was a really simple idea, most of which had been done before, just not all on the same site. The reason they were successful wasn't because of the strength of the idea, it was because they got their site out there when the only real competition was Friendster, and since their site relies on network effects for much of its usefulness, it has been difficult for others with the same idea to be successful.
She screwed you over, but I doubt there's much you can do about it except try to beat her. Also, people who do things like that should have to pay with their reputation, but if your idea is too obvious, people will just think you're lame when you tell the story.
http://journal.dedasys.com/articles/2007/04/26/ideas-are-worthless
With that said, I still think he should try to compete with her if he thinks the idea is worthwhile, but he should definitely reevaluate whether or not it's worth it to leave his job/school to do it full time. At this point, anything less than full time isn't going to accomplish anything.
Back in '01 when my uncle was doing his startup, couple months before his site's launch we ran across a site that was VERY similar to what he was building. And here was a site that looked like it had been born out of my uncle's business plan. Few months later that site would fold--and we're still in business today.
Better question is HOW well is she doing what you planned to do.