Leo says it really depends on how the disc was burned. It's likely the disc has gone bad. There are two ways a disc can be burned. One is to burn the whole thing and finalize it, meaning it can't be altered later. The other is using UDF, meaning he would be able to add to it later. He would need a special driver to read that, and he'd have to finalize that disc before it could be read by other computers.
Since he burned it using Windows Photo utilty, that's probably not the case. If Bob copied the photos to DVD and then deleted the originals, it was inevitable that he'd lose them at some point. DVDs can, and often do, go bad.
It's always best to use a 3-2-1 backup strategy ... three backups, using two formats, and one off site. Check out DPBestFlow.org for more tips on digital asset management.