We have a neighborhood watch group and if it happened to me I'd post what was happening and let the neighbors draw their own conclusions. We try and look out for each other and the watch group has been a real help and the police are thrilled with it. If anyone even comes to our neighborhood whose car is not recognized, someone calls in the tags into the police. If they are not a criminal they have nothing to fear. We had an 80+ year old woman beat badly and car stolen. Since then we have swung into action.
He does not have to live like a hostage in his home. He has committed no crime if he lets others know that he suspects his dog may have been poisoned. I'd be very grateful if a neighbor told me that since I have pets that also occ go outdoors.
So yes he can tell other people like the neighbors or vet . He can't prove it but he has strong suspicion the dog was poisoned and fears for his little girl and he thought the neighbors might like to know that someone may be poisoning the animals. He's not saying who, the neighbors probably already know more than he is aware of.
neighbors should stick together and protect each other.