I've tried to convert some slides in the past with scanners and wasn't too pleased with the results. Next time, I'm thinking of trying something like ShotCopy. It uses your digital camera to take a picture of the slide.
Are you talking about a regular flatbed scanner that most everyone has, or one of the dedicated 35mm Film Scanners like I suggested for him to use?
I have 2 wolverine film scanners. I used them to scan my grandfathers slides. I was happy with the results. Look them up and I'll be happy to tell you more about them.