Well, it is a dangerous place in some ways but I wouldn't say those are the most dangerous things, especially if they're well-managed. I would want to know who is organising it check out their safety policies and history, but those things sound like the kind of thing that thousands of tourists do safely. The ziplines are everywhere and very safe. The volcano isn't a worry - it will be one of the safe ones with tourist paths and wardens. White water rafting should be OK if they follow all the safety rocedues and trainig, and horse ride sounds OK. There are some nasty snakes and so on but they're rare, and anyway you'd be safer on a horse.
One thing he should steer clear of if possible is surfing/swimming in the sea. The only horror stories I heard involved the dangerous rip tides and currents. Just in case, get him to read up on rip tide safety.
I didn't find the mossies that terrible, but I did get bitten - I was advised to take anti-malaria medication and I did. There are actually smaller biting flies that are more annoying and although it's hot, he should take long-sleeved/legged clothes for evenings when the beasties come out. Also a mosquito net and your own lightweight sheet are invaluable - wherever you sleep, you can cover the bed with your sheet and tuck in the mossie net all round, and no creepy-crawlies will get you.
On the plus side it is absolutely amazing and beautiful, and should be a fab trip.