The simplest way forward will be to approach the owner as a neighbor and concerned citizen. And offer to help with advice on living conditions, medical help, socialisation, etc. If approached directly, politely but firmly most people will both engage and give you access to the dog.
That's also the best way of assessing what's really going on.
We get many requests to help but the reporter does not want to be a part of the engagement -- unfortunately, it doesn't work like that. Even if a crime if being committed -- for instance, cruelty -- for it to stick you have to engage because no one but the one who sees it has locus.
If you need advice to give the neighbour, we're happy to help with that.