Lots of sizzling/fingering along for that kid so their getting the mental process, but not making bad noises that distract other kids.
Have the entire class go back to those first few lines of the book, but with new goals of tone, balance, blend and precision. You can also play them faster. It may also qualify as a sight-reading exercise (if it's been long enough). While the group does that, you're standing with that one student, giving quick tips.