Well it's quite possible that the root cause was worms. It's not uncommon, and that would certainly fit with the green frothy droppings.
Say, if your dad is a vet, you could have a fecal egg count done. Animals won't always shed worms if they have them. The worms perfer to stay up in the animal and steal their food! But they WILL shed their eggs, and a fecal check will allow you to check the droppings for the eggs - which gives you an idea of what he has, how much he has of it, etc.
If he has a lot, I'd recommend the weaker wormers first - piperazine, etc. Then a followup worming in 2 weeks with the stronger wormer like ivermectin, fenbendazole, or levamisole. That way you don't overload the bird with paralyzed worms all being sloughed out of his system at once.
If you have other birds, worm them all at once - mark your calendar so you'll know when you did it.
If it's been a while, and you had another case of it somewhere in the flock, I'd definitely say it's a strong possibility. Good thinking!