Unless you're name is Cesar Milan, or you otherwise have a strong background in dog training, I don't think you, as the non-owner who's with the dog only every other week, are going to be able to humanely stop to it. Truly wish I had a better solution for you.
PS please note I'm not saying it can't be stopped at all. It probably could be, but it would take a consistent, concerted effort on the part of the owners, including dgd, every day, as well as a very good trainer to get them started. It sounds like it's pretty ingrained now.
FWIW, I couldn't stand it, either.
You may have people tell you to use a bark collar, or a citronella collar, or a spray bottle of water, etc. While you could certainly try those (personally I would not use a bark collar under these circumstances), a) IMO it's a bandaid solution, the real solution being teaching the dog not to bark; b) I don't think your dgd would "allow" it (not that I think 13yo's should be making the decisions, but I think it could cause her to not want to come to your house if she thinks you're "hurting" her dog); and c) (and admittedly this probably falls into the "who cares" category for many people), it's unfair to the dog to do something like this without teaching him what you
do want first, IMO - this really needs to come from his primary family.
Good luck. That's a tough one.
