One in 4 is a lot to develop gular edema.
Maybe he should consult a vet about it?
I would also say that your friend should look at what is being fed to the chameleon, dusted on the insects and in the things that are fed to the insects to see if there is something that could be causing it.
Here's some information about it...
http://www.allcreaturesvetcare.com/lizard_chameleon.php
"Excessive vitamin A supplementation may result in organ toxicity (kidney and liver) some times causing gular edema and reproductive problems such as sterility."
"Excessive vitamin D3 supplementation can result in organ toxicity (gular edema) metastatic calcification, gout, and pseudo-gout (calcium-like deposit)."
"Treatment for hypervitaminosis is more difficult as organ toxicity and damage has usually occurred by the time clinical signs have been noted (e.g. gular edema with kidney damage). Aggressive fluid therapy (see therapy for infectious diseases), exposure to natural sunlight, providing a lower protein diet and using phosphate binders such as Amphogel may be helpful. If hypervitaminosis D and metastatic calcification is suspected."
And another site....
http://www.all-creatures.com/site/view/79378_Chameleons.pml;jsessionid=wla3ankdcg7