I board dogs and have tons of close neighbors. I can't have barkers. I've had to do a lot of bark training. My associates and I tried all the bark collars, shake cans etc, then we found out about squirt gun training for obsessive barkers. You get the little 3 pack of small Nerf Zip Fire super soaker pistols off Amazon. They even come up as an option for dog training if you just search for squirt gun. They're fun for moving squirrels along out of your trees too.
The key is to be as surreptitious as possible, so the dog doesn't really know where it's coming from. It doesn't hurt the dog at all, just gets them wet, but the point is they don't see it coming, you say "Quiet!" or whatever their word is, and fire immediately! Suddenly their face or ear etc is wet, and they are startled out of the bark reflex. Next bark, instant zap, until they generally stop out of pure puzzlement or disgruntlement from having a wet head and chest or shoulder area. They often just go lie down on their bed.
The owner should squirt, since the dog is looking at you and you are handling the guitar. Try to disguise that the squirter is holding anything, even if it's from under folded arms or from the hip. If they learn what you're doing, the really smart ones know they can bark their heads off until you get the gun and point.
I can have 5-6 puppers in my patio and yard (always supervised for their safety), all lounging on their rockers and outdoor couch, or laying out in the sun, and 2 or 3 pack piled on me all cuddled up, and someone walks by and even though they can't really see the pups well through our privacy screening, I say "SHH!" and can then have a conversation with my neighbor through the fence, and no pup does more than look at the passerby, and generally nary a peep. They can even hear other dogs barking next door in several other units, and they are the only ones who are quiet. They can't be bothered. They have become Stealth Doggos. It sometimes takes a few sessions, and even when outside I can get the new to the crew kiddos 20 -25 feet across the yard. None of my regulars bark. Barking is often a mindless reflex for dogs. Sometimes it's out of fear, but using a squirt gun is operant conditioning, so it usually works either way. Terriers are pretty fearless so I'd guess not in this case, and the training is a very specific situation here.
I want my own dog to alert bark, but not be obsessive. We don't worry about a few alert barks or barks when playing. It's the non stop or inappropriate stuff that isn't tolerated. Dogs are pretty smart, terriers are smart and extremely willful.
https://www.amazon.com/dp/B00EAHXWPY...516004377&sr=3
https://www.amazon.com/Soaker-Zipfir...r+dog+training