Hmm, practice is golden, however I think he might need some more directed advice on this one.
First of all, you say that your pinky is holding back a card while executing the pass. This is because while you're doing the pass, you're not lifting the top packet off of the bottom one, you're simply sliding it, causing your pinkie to scrape the bottom card. While it's definitely not a hinge motion, you should still separate the two packs before moving.
Concerning why the top packet flips, I've been sitting here with a pack of cards for 5 minutes and I still can't even fathom how that's happening
A video would really help