He should be able to do just fine with iMovie (editing the film) and iDVD (putting it on the DVD, "burning it").
I've done this on a Windows platform, not Mac, but I've also read that iMovie is simple and effective. Perhaps we can ping you some help. Or maybe you could google "iMovie" for tutorials.
Actually - if the video to be edited is already on a DVD (in DVD format that is), then it will have to be "ripped" to a format that iMovie can use (QuickTime or DV). From that point, it's quite easy. Complete the project in iMovie then simply click "send to iDVD" if you want to put it back on DVD. I would recommend NOT putting back on DVD, instead clicking on "share" and save as a web-friendly format.