Non stop barking in elderly dog?

I'm sure someone who knows more about dog behavior than I will eventually chime in, but in the meantime, have you contacted his owners? They may have some insight.

That's where I would start.

(And put him in an inside room to contain the sound.)

Oh, I feel for you! :-/ Unfortunately, your options are limited. If you don't have permission from the dog's owner, you shouldn't be giving Benadryl, or any other medication, and you shouldn't use an electronic bark collar (especially since it isn't sized for a small dog). It's possible that the dog has age- or medically-related brain issues, like senility. It may also be that the dog is "off" due to being in an unfamiliar environment.

One thing to try is some sort of soothing or distracting sound (ocean wave sounds accessed through your phone or computer, soothing or distracting music, TV, etc). Another possibility is to take him for a long walk (if he is able); maybe if he's more physically tired, he'll be more calm.

In the future, it might work better for the dog if the owners used a traveling sitter, rather than boarding at a sitters house. The dog would probably feel better if he was in his own, familiar environment, where he doesn't have to learn a new house layout, and has familiar smells around him.

I sure hope the owners get back to you soon and have helpful tips.

He is an older dog, probably not feeling well since he is diabetic and to boot his eyesight is limited. I noticed you have several pets in your home, he is most likely anxious due to an unfamiliar place with other pets around that he can hardly see. Older dogs in my opinion should be with a traveling sitter as Shannon suggested, so he can stay in his own environment. You can tell the owners that when they pick him up. In the meantime I would suggest you call Rover and tell the what you are going through. Sometimes they have more success in reaching the customer than we do. If he is physically able, take him for a long walk to ease his stress and tire him out. I feel bad for you but I feel bad for the poor dude to.

I'm glad the owners were able to make other arrangements and that you're no longer having to deal with it.

I just want to reiterate one of the things Shannon said - you should never give a dog anything not approved by the owners, including food, treats, supplements, and medications. Especially when the dog has other medical issues. And even more especially for off-label use in treating behavioral concerns, like the sedative effect of Benadryl. Medicating someone's pet, even if you have experience with the medication and its use in canines, puts you in very dangerous waters. Could you fathom telling an owner that you had to take their dog to the emergency vet due to a medication reaction after you gave them something to get them to stop barking? I've definitely joked about drugging a dog at 3 am when I haven't slept in 4 nights, but that is a conversation I could never have with an owner.