Puppy with giardia

My sister recently got a puppy. They found out shortly after getting her, that she has giardia. She was told by the vet to purchase Forbid, I'm assuming from them. She bought it, mixed it with her food, but apparently Attie has found old poop laying about and ate it, reinfecting herself. Does anybody have any advise for my sister? It would be greatly appreciated! Oh, Attie is a Cairn Terrier. Thanks!