New Neighbors feeding my dog

Okay, I need some advice.

My husband and I have lived in our home for around two years. We have awesome neighbors, but one of them is a little grouchy. But, we get along pretty fine with him. We just try to mind our own business.

He has a pool house that he’s converted to an apartment and rents it out.

He just had some people move in recently. I’ve noticed my dog throwing up a bit lately, but not a crazy amount. Today though, he came inside from using the bathroom and threw up roast beef, potatoes, and carrots. My husband and I walked the fence line of our property, and sure enough, someone has poured their dinner out on our side of the fence.

My dog has an extremely sensitive stomach. It took us around six months to find a good dog food for him that wouldn’t make him sick.

We haven’t ever met our new neighbors (the renters) and we can’t prove that it isn’t the home owner. But, we haven’t had this happen in the two years we’ve been here until these people moved in, at the beginning of December.

What is the appropriate action to take? Do we talk to the home owner, our “real” neighbor, or do we confront the people renting the pool house since they’re the ones doing it?

*edit to add- this is not the first time our dog has been out playing in the yard or outside using the bathroom and then come inside and thrown up. We always assumed he got too hot/drank too much water too fast when he came back in/something like that. This is the first time we’ve been able to tell that someone is actually feeding him. I took this picture, and you can tell that they just dumped it all...