Christine is having trouble burning DVDs via iDVD. Leo says that program hasn't been updated in years. Apple abandoned it, but many still use it. If Christine updated it, that could have broken the usability of it. It could also be a bad DVD blank. The bottom line is that the future is calling, and optical media isn't coming along with it. We're moving to the cloud. But for sharing movies with family members, a DVD is still a good idea.
It also could be a dying DVD burner. Leo suggests checking out the trial version of Adobe Premiere Elements. If she can burn to that, then it's a software issue. If it still doesn't work, then she'll have narrowed it down to a bad DVD burner, and those are pretty cheap to replace at around $30. She should get a USB external drive from Asus or Samsung. They work with Apple hardware as well.