I will repeat what so many others have said: It is always a bad idea to mix species. They just have so many different requirements, and have different toxins that can be deadly to each other. It's tempting because It's fun to have different species, but it's not healthy for them.

Also, the coconut fiber shouldn't have caused impact. I use coconut fiber for all 8 of the Whites I have had in the past year, and impaction has not yet occurred. They rarely swallow any, and if they do it's a minute amount and ground fine enough they just process it through their system. Unless your frog was eating mouthfuls and mouthfuls of dirt, I doubt coconut fiber was the culprit.

Question: Is your vet a reptile/amphibian/exotic animal vet? I feel like a lot of vets might try to help out an exotic animal and basic research does show impaction can happen, but I feel like an experienced exotic animal vet should and would know that finely ground coconut substrate shouldn't cause impaction. Yes, we are strangers on the internet, but consider that many of us have years of experience with this species (myself, I have over 20) and while a vet may mean well, they may not have their own direct experience with a species, even if they are an exotic animal vet. Personally, I trust my vet, he's an exotic animal vet, but I also give a lot of weight to those on here who do have a lot of years of experience keeping Whites, the fact is sometimes they know more than my vet because of their direct experience, and because so little is known about these guys and their illnesses. Just a thought. Through vet visits and online research and forums I have been fortunate to save several little guys from some pretty dire health situations to have them go on to lead long and happy and healthy lives. Out of all 13 Whites I have ever owned, only 2 have ever died from illness, all the rest have gone on to live long lives and appear to have died from old age.

Also, mixing species won't cause instantaneous death, but the toxins can slowly accumulate in the frogs' system and cause death that way, just FYI.