I'm glad you got your money back and your other frog seems healthy.

It's hard to tell if a frog is sick. Sadly by the time they look sick they're usually extremely sick and it seems like that was the case this time.

I've heard if you buy a frog it's important to examine it first and then put it into a quarantine tank so you can see if it's eating and moving normally before you introduce it to your other frogs.