Hi Michael! Sorry your frog is not well. Without a fecal it's not possible to tell of any parasites. A herp vet or online lab can do those for you.

You could separate them and set-up the skinny one in a hospital like enclosure. That will make collecting the poops sample easier and separate from each other. Kind of late to ask; but did you quarantine them? Since they where housed together; I would medicate both for whatever condition you find on skinny one with stool issues.