Hi Eric,
I had a bike like that once - the chain used to fly off !!
Obviously like my bike , something with your mates set-up isn't working correctly, and if they did that to me, I wouldn't use the bloody things.
For me, they're good for pulling on the string ,but I suppose you would need to be careful on the down stroke , and maybe they're unsuitable for the autoharp when flailing up and down , back and forward.
I tend to use them just for a thumb pick to get a bit more sharpness out of the bass notes ( on a normal acoustic guitar ).   
Come on Eric there's got to be a reason ,the right tools for the right job. Your mate probably needs those thumb picks which come out from the side , besides as I said to Doug. " ..No personal product suits everyone " and apparently neither does this.
Cheers