I'm sorry angel
I have always read/heard that frogs are generally disease free and that they do not easily pass disease to fish. Perhaps there was an ammonia/nitrite/trate spike? I've heard of that happening when new frogs are introduced because people aren't sure of how to feed them exactly and food ends up sitting in the tank. Maybe the betta didn't like the frogs and got stressed...
At any rate, I'm sorry
I told you it was probably just shedding its skin (happens with my frogs a lot and it does look just like fungus)...not sure what did happen though