What to do about my neighbor’s poor dog?

I have a neighbor behind my house with whom I share a fence between my backyard and theirs. There are a couple spots in the fence that are in the state of disrepair, but neither of us have bothered to fix it. I have two dogs that are too large to get through the holes so it’s never been an issue.

A few weeks ago my neighbor got a puppy. It’s a large breed puppy that I would guess weighs about 40 pounds. The puppy has been making its way into my backyard daily for the last two weeks. I have no issue with this because I think my neighbor mistreats the dog. My neighbor went on vacation all of last week and left the dog in the backyard. I even went to knock on the door to make sure they were gone. During this time it rained and the dog was left to fend for itself outside. There is no dog shelter in the yard. The poor dog cries all night and keeps people in my house up.

I don’t even want to repair the holes in the fence now because I feel so bad for the puppy and it enjoys playing with my dogs in my backyard. It gets no attention otherwise. I’m home all day and the dog spends 24/7 in the backyard only visited by the owner when he puts out food. I come out a couple times a day just to pet the dog and it’s seems so happy when I do.

Would it be beyond my place to talk to my neighbor? Maybe write them a letter? Normally I wouldn’t get involved, but having to listen to a sad dog cry loudly and daily for the last several weeks has been heartbreaking.