An overabundance of oxygen in the water is not a factor here, even if it might possibly occur for some unknown reason, unrelated to the pothos. Dart frogs live on land and seldom go ino the water, if at all.
Did the bloating occur before or after the frog died? If before, it could be related to numerous other health problems, such as parasites, kidney failure, (reasons not easily discovered)or something in the environment that caused an electrolyte imbalance in the frog-- all very rare events if the temps and humidity are right. Since darts eat with only their tongues, they rarely get gut impactions of foreign material like the grab and gobble amphibians are prone to do. What kind of water was your friend misting with and was it a lone frog, or are there healthy companions still in the same tank? Could the frog have sustained an injury of some sort? A newly introduced frog may leap around and perhaps injure himself on a sharp object.
If it occurred after the frog died, that is just what happens to dead bodies shortly after death. They bloat.
Without a herp vet to do an immediate necropsy, it's impossible to diagnose the problem accurately.
If it's a fairly new frog in a new tank, I would contact the breeder and tell him or her about it and see what suggestions he might have. While at this late date,(they usually gurarantee only safe and healthy arrival, and after that, you are on your own,) he won't give your friend his money back or replace the frog, any reliable breeder will talk to him about it and make suggestions.
P. terribilis are usually very hardy frogs and easy to keep with the proscribed conditions.
Do a check list of everything, from the temperatures (between 65 at night and not over 80 during the day,) the humidity (80 to 100%), food (plenty of fruit flies dusted with calcium/D3 and vitamin supplements) substrate quality (no pesticides or fertilizers in the substrate,) cleaning the glass (no Windex ammonia products or chlorine,) room air-- no insect sprays or aerosol air fresheners, new enamel type paint, petroleum varnishes applied recently, etc.
Sometimes it's a pain in the butt to figure out why a frog has died. Be encouraged, that it is generally a rare event under suitable conditions.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus