I am very sorry to hear that your frog passed away. D: It is very likely that whatever ailment he had was stress induced because of the harsh environment he was subjected to at PetCo. Don't feel bad, you did everything you were supposed to do, I promise. D: All you can do now is watch your other frog very closely and see if he starts exhibiting the same symptoms. If he does look like he's starting to deteriorate just as the other frog did, I would strongly recommend you take him to a vet and make sure you stop whatever ailment caused the other one's death before it's too late.

I wish you luck and my condolences for your loss. <3