I suppose anything is possible and yes, lightening does sometimes strike people . . .
Cappers were originally designed as a way to easily carry caps and quickly install a cap on the nipple . . . mechanical cappers have been available since just about the creation of the cap system. During the Civil War, cappers were available for the rifled musket but were not an "issued" item.
Cappers were available as part of cased Colt sets . . . but for the majority of users, caps were and have been installed by the good old method of using your fingers. It's documented that someone who was well versed on loading a '51 Navy could quickly lay three caps in the cap groove on the frame and rotate the cylinder and install in a matter of seconds.
I'll be the first in line to promote safety . . and if you want to use a capper, that's great. Personally, I hate 'em. I've never found one yet that works well for any of my revolvers and the only time I have used them is with a single shot rifle - and only as a "convenience" of capping during hunting when fingers are cold or on the line when shooting competition. For that, I use one of Ted Cash's.
If he guy says he had a cap go off . . then I'll take him at his word. But I'd also like to see his pistol and how he was installing it . . were the nipples in good shape or did he have a burr on one? Was he trying to force an undersized cap on an over sized nipple? What brand of caps? What's the complete story?
I have used and installed literally thousands of caps on nipples over the past 50 years . . . rifled musket, rifles, shotguns and revolvers and have never had an issue. But, I have also always made sure that the caps fit the nipples and the nipples were in the condition hey should be. Installing a cap on a nipple probably requires far less pressure on the cap than a primer requires when being seated in a casing.
In the long run . . . yes, it could happen . . . chain fires can happen as well so it's probably prudent to only load and shoot once chamber at a time. I'm not making "light" of this but if you use commo sense when loading - there shouldn't be a problem. In my experience with a wide variety of mechanical cappers, I'd be more worried about a cap going off with one of them trying to get it worked on to a nipple because there wasn't enough clearance in the cylinder cut out for it to fit properly.
But hey . . . what do I know? I'm old and crotchety . . .