We feed raw too and when I read the canned horse meat I was thinking....well, it has to be full of preservatives to keep it fresh right? I would assume so since it's canned. I would say that most raw feeders are all about fresh or frozen meats and you won't see them going the canned route as it doesn't contain bone and I would think that poor little dog probably has constant runny poo.
I would agree with what the other posters have said, best to stay away from that route. Just point out to your neighbor what the can says.
I think I would handle it like @GatorRay said. I would go out of my way to see him again only this time I'd start the conversation with something like "I was reading that can and was wondering, since it's a supplement only, what else you were mixing it with".
He may not know it's not really a whole food but he seems like the kind of guy to get offended of you point it out to him. This way you kind of flip the light switch on for him without hurting his feelings or getting into an argument.
For me, her name was Abby
10/24/2011 - 11/23/1012
Obtaining a dog license should require more than writing a check.