They're not social. Not sure where you got that from I'm afraid. It sounds to me like the skinny one has succumbed to some external stress and now has a stress-related illness. The fact that it tries to eat is a good sign but if it can't keep the food down then you really need to get it seen by an amphibian-savvy vet I'm afraid. They might try dosing the frog with flagyl to purge its gut of any parasites (which won't help) and it will also stimulate its appetite. If the vet thinks the frog is far gone then he/she may try dosing it with antibiotics.