Just a thought, if you just got this dog and he was malnourished, he may just be stressed out for being in a strange home with another dog. If stool and urine and vomit are ok, just observe and report! Everyone just calm down, this guy is just worried about his pup and looking for reassuring comfort from guys he feels he can trust. He's probably already very attached to the dog an doesn't want it to be ill. I honestly think it will be fine, just make sure it has constant access to food and water. Maybe quarantine it so you can tell if it's eating.
The difference between stupidity and ignorance. Ignorance is doing something wrong and not knowing better. Stupidity is knowing better, but doing it anyway. Don't be stupid!