My 8 year old housebroken German Shepherd stays with friends when I travel but he goes to the bathroom in their homes, no matter how much they walk him. How can I correct this?

Is this dog crate or kennel trained? Does he have a command that you use to prompt him to go to the bathroom? The very first thing I teach all my new puppies or even older dogs that have joined our family is to use the bathroom on a leash and on a specific command. I use "hurry up"

Either way, since he's only doing this there I would suggest he always is on a leash or kept in a baby gated room, crate or kennel. I would not allow him the freedom to wander around the house.

Since he only does it at their home I doubt it's a medical problem but to be sure you could have a vet check him over. I know some dogs will "hold it" when they're in a new place and previously housetrained dogs will then have accidents. Maybe you should take him over there randomly between trips and go on some walks with him in their area or yard?

Maybe a pet sitter would be less stressful for everyone, especially as your dog gets older. Since you have a fenced in yard it may be the best option for the dog's well-being and to maintain your friendship.

It's not really possible to correct this if he won't do it in your presence so I'd still always keep him in an area where he's contained. You've got some nice friends!