Yep, I suspect this going on , intermittently and very occasionally , with Rex too. With her it settled down in a minute or so when I give her a snuggle and pet her.
I thought there might be link with colder weather (most the times we've seen this her it's been overcast / rainy and chilly (outside), or maybe coincided with a change in Ca dust from AristoPet ReptiCal to Zo0-Med Repti-Calcium.
The thought also crossed my mind that I might have ODed her on large silkworms. Too much Ca ?
Or she'd had too much buk choi and puk choi in her diet ( vitamin OD ) ?
I had also not long before we first saw her shivering , changed over from her old pendulum style reflector dome which she tended to get very close to (under 2" from the globe), with a nanohood mounted under her enclosure's solid lid (so she was further from the UVB than before the change).
So first up : I checked the install date on my old UVB source (I use permanent ink markers to date these with installation date on their base) , consequently - I recently swapped over her old 13W UVB200 (installed in mid August 2015 and run 16hr / day since), with a brandnew 26W UVB200 , I'm hoping this will help get rid of her shivers.
She certainly seems to love her new UVB globe, basks most the day directly under it (rather than under her basking globe (a 80W par38 Philips spotty) , and demands to go back to bask (even if offered a snuggle or time on the floor to go freerange).
I may start giving her liquid Ca (CalciVet solution by mouth, since it's hard to get a uniform Ca dusting on insects and this make dusting them a bit hit and miss. She's been off her greens lately too.
I'm hoping to see some improvement in the next month or so. Is a month long enough to see an improvement in blood Vit D in an adult beardie ?
I hope your friend's pet Puff gets an improvement in his UV lighting and comes good soon, might be worth telling him it may take a little while for an improvement to be apparent , depending also on how much his condition has developed and how much of a Vit D and Ca deficiency he has.
Tell him he needs at the very least a UVB200 globe (possibly a 26W version depending on the height of his enclosure) and it needs to be places close to his basking light.