It's not a dart frog, it's a red eyed tree frog. They only eat fruit flies when they're really small, otherwise small crickets should be sufficient. The cricket I can see in the one picture looks to be the proper size. Had he been isolated in a quiet area for the whole week? Or she just moved him? If she just moved him, give him a couple more days before worrying. He's thin, but I don't think he's in a danger zone.