I've never had or heard of someone having that clicking in the spine... however, I used to have that clicking in one shoulder when I would do snake arms. I don't remember when my shoulder stopped doing it but it did. I'm thinking that it's the same principle... of course, being that it's in the spine, it's a little bit more worrisome in general than a shoulder but I think that, as long as her posture is correct, it's fine and will go away.

You may want to post your question in this tribe too:
tribes.tribe.net/healthybelly

That's never happened in my classes or to me, but when doing certain exercises (leg lift type things, while laying on the ground) my bones will click or sometimes pop out of place where my leg meets my hip. It's weird, but it doesn't hurt anything. I think I'm double jointed in my hips somehow.

Does this girl have any pain when it happens? It could be just her back popping, although I've never been able to pop my back just doing chest slides (I wish I could)

Honestly If there was anyone who could figure it out, I'd ask Suhaila. She has a wealth of information about the body and has taught so many students, she's bound to have had a similar issue at some point. But if the girl is worried about it or if it bothers her, it might be worth seeing a chiropractor.

It could be tight tendons snapping across each other, or she could have instability in the spine and need to strengthen the area. She needs a professional (sports medicine or physical therapist to determine this, and proceed from there with exercise for the particular problem.