My African Dwarf Frog died from a fungal infection?
I bought some stuff called Lifeguard All-in-one treatment…it is actually for fish but the pet store employee said it should work for my frog too. Was it a bad purchase? Should I get something different for my other frog (who appears to have a fungus too)?

I don’t know what kind of fungus it is, but look up chytrid and see if it looks like that. If it is, heaven help you and your frog. You can treat it but it is hard.
If not, you can try gradually raising the tank’s temp to 89 degrees over a period of hours and keeping it there for a day or so to try to kill the fungus. In my experience the over the counter meds don’t do much good.
ha that sucks