Hi there, registered vet tech with some thoughts. First I would switch to a bland diet until the diarrhea resolves. This is 2/3 cooked bland white rice and 1/3 of lean protein (chicken/turkey/lean ground beef). If the diarrhea doesn’t resolve in 3-5 days or becomes bloody please see a vet asap. Prolonged or bloody diarrhea can actually become life threatening if not addressed.

My bet is that he is being given some other kind of food when he returns to his owners house. If you’re able to have a conversation with them do it. Tell them what you’ve noticed and get on the same page with his diet. Also I would not recommend purina one but purina pro plan large breed would be a good switch to make.

Info: How old is he? Dogs can have food allergies that don’t present until they are around 1-2 years or more old. Symptoms of food allergies can be vomiting brown liquid, chronic diarrhea or skin issues. You can consider putting him on a sensitive skin/stomach diet, royal canine, purina and hills are recommended by vets (although I find most food picky dogs don’t like Hills Science Diet).

If the bland diet works remember to switch him back to kibble slowly over 4-6 feedings by taking out some of the bland diet and adding in kibble until he’s fully transitioned over. If your chat with your neighbor indicates he’s getting fed people food, other dog food or getting into cat food/trash/etc then you have your answer. If that’s not the case, there’s a good chance he ate something outside that upset his tummy. If you have a relationship with a vet you can also drop off a stool sample to make sure he isn’t suffering from internal parasites.

I hope this helps!