When a dog owner leaves a dog with you and the dogs poops are always loose and you attribute that to inadequate nutrition in the food they left, How do you address this with the owner?

If you know for a fact that the food has been recalled, I would message the owners with a link to the recall notice. Tell them you are concerned that they may not be aware of the recall and ask what they would like you to do. Offer alternatives - you cook something, you go buy something and they reimburse you. Be tactful and approach it in a helpful way - "I don't know if you're aware, but xxx brand dog food has been recalled."

If it's not of high quality and the dog has loose stools over several days, also message the owners (if it's a long stay) and say something to the effect of "little Rover has had loose stools the last several days and I wanted to check and see if this has happened before. I have tried cooking chicken and rice with other clients and this helped solve the problem. Would you like me to do this or do you have an alternative suggestion?" Again, I would approach it as being helpful and asking for their input rather than making the decision on your own and telling them about it later.

Loose bowel movements could also be at east partially due to stress being in a new environment. However, I understand your concerns with the food provided. Ideally, during the meet & greet, you may have already found out any such issues and what the owner allows. I have cooked & cooled chicken and rice when needed and also other allowed items. Afterward, I'll inform the pet parent of what I did for their pet to solve the issue and status. If not pre-approved depending on the urgency, I'll let the pet-parent know what's going on and ask them for their tips for their dog. When the owner returns for Rover, that might be you best opportunity to see you enjoyed watching their dog, what you did, and then bring up that the dog had loose bowel movements. Wait and allow them to respond (awareness or surprise of problem). Then, you can share what you know about dog food and maybe how you learned about it with tools they can reference (spoken from a helpful perspective of maybe they just don't know).