Last comment because I think it's worthwhile to others to think about this if they find themselves in a similar predicament, or even take pet sitting on as a business, etc. Since this is a thread about pet sitting, I think it's appropriate to discuss in a general manner.
I am also in the camp where I believe if things get too much for the older dog, she will correct the younger dog but I am always there so it has never got to that point.
As a "pet sitter", it's not your place to let this happen or even potentially happen: your job is to keep the dog safe while the owners are gone. I'm getting the sense that somehow the lines were blurred here, and it's not meant as an insult to the OP - clearly she is a very giving person and for that she should be commended. But many of us probably also do this when we're watching someone else's pet(s). It's well-intentioned, but here's why it's misguided.
We don't have to necessarily incorporate a dog into our household when we're watching it. We just need to provide it a safe place to stay and meals, shelter, water, etc. We need to remember that if the dog was in a kennel, this is how it would be. Here, it's a little less stressful without all the scents and barking of a kennel, so it's preferable, but it's still a job.
When you have the jaws of a large dog vs the body of a tiny dog, even a "correction" bite could be disastrous for a little dog and result in hundreds of dollars worth of vet's bills and more. It's akin to leaving your toddler in a room with an older bully with a club and hoping he doesn't hit your child with it because you're nearby. Safety should be the #1 priority. This tiny dog should not be allowed to jump on your GR at all. In fact, they should mainly be separated unless the visitor/intruder/interloper (in resident dog's eyes) is well behaved, which, IMO, a six month old puppy will probably not be (especially in a home it's visiting). *Maybe* some supervised play time outside together, where the GR can get away from the pup if he's getting annoyed, but I'd even suggest that play time as the only dog would be preferable in this situation.
In a home, where there's little opportunity to get away, it can be tough for a dog being jumped on and bitten by little sharp teeth constantly. Many dogs in the same situation would give a warning growl, and if the pup continued then, a bite. ANY dog can bite given the right circumstances, and these are classic bite circumstances in the dog world. I wouldn't want to have to tell returning owners their dog was injured on my watch. Little dogs like Yorkies have high injury rates because their bones are so small and they can snap easily, etc.
Bones are necessary for a retriever's chewing tendencies and we do have them all over the house but my dog regards them as low value and never has guarding tendencies. She just goes and finds another one when the puppy takes them. Toys are also low value to my dog so she doesn't guard them either. You may think that is bad training but it works in this dynamic.
He can have all the bones he wants when there are not visiting dogs around.
Look, I know you have a great GR and I have no doubt he is a wonderful dog. But please know, he's a dog and he has instincts and will act on them given the right circumstances. Friends of ours also have a wonderful GR who is as gentle as a lamb and around the age of your dog. About a year ago, when my dog was just about the age the dog you are sitting for now was, he was sitting with this GR for quite a while and they were getting along famously. I didn't realize it, but someone gave the GR a rawhide and next thing I knew there was a ruckus when my dog went to take the rawhide from the GR. Fortunately, neither dog was hurt. These two dogs still get along great but we don't allow treats in the mix any more. This was something I practiced long before this incident, too. Other friends of ours had a child severely injured by a dog when the child went near its treat. (And the dog paid for that with his life.
) Many, many dogs will cross that line over a treat. Common dog sense just dictates that an ounce of prevention is worth a pound of cure when it comes to bones, rawhides and treats with dogs.