I reccomend doing 50% water changes every other day or so to keep the ammonia down until it has disappeared entirely for a few days.
Is the betta the one in your profile pic? He's so beautiful, I love that colouration Best of luck to you and your fish!
Likely a fungus. The chytrid fungus has infiltrated the adf breeding stock. Other fungi is a possibility too. It does not affect fish.
Wait at least two months for the spores to die off before purchasing any more frogs. I would highly recommend putting new frogs through quarantine and treatment for fungus before introducing to your main tank. I used the lamisil method which seemed much easier than the BC in the links below.
They are social so you should keep at least two by the way. Did you mean that you had two and there is one left? If so, I would jump on the treatment right away and keep it out of that tank for two months. Let me know if I can help in any way.
I will find and add my thread where I went through this with four new frogs and almost lost my original two before I learned about it. The lamisil treatment saved them. I will use it when I am ready to buy some again. I also have the the BC on hand now too.
EDIT: here's the link:
African Dwarf Frogs Dying
Sorry you lost your little guy.
I was really sad about my four new ones, and would have been devastated to lose the two I have had for 2 years now. Just upgraded them to a 20g L so I could have six and it almost cost them their lives.