He's probably rendered the file larger than the DVD/project can hold.
More info would help. How long is the project? Bitrate he used? AC3 or PCM audio?
>, if I press the "Optimize" button, it will automatically re-render everything. Don't press that.<
File>Optimize DVD opens a window which, among other things, lists the project assets and if they are compliant or if they need to be encoded. It will not set anything into motion. That would be part of the Prepare and/or Burn dialog.
Maybe I need to recheck, but I never need to press that button. The information is there to see as soon as I start the process. If I need to squish the mpeg even smaller, then I press that button, but the information is already displayed, all the warnings, the size in bottom right corner. The percentage of media to be used. It's all there, without touching the button. So, tell me. What's that button for, if it's not to squish your mpeg smaller?
EDIT: Okay, I just opened my "Optimize" button on my DVD-A1.0 and one of the 1st things I noticed, besides the bitrate it would re-encode with, was a gray box with a check in it, that says "Recompress video". The project only had 96% used, yet it looks like it will recompress if I continue.