That's your answer as to why pal, 1 dog at a time if your wanting a retrieve. Even holding 1 on a slip whilst other is running causes probes, running doubled up also messes up a good retriever. I know it ain't easy but get a lot of 1 on 1 with the dog. If your birch is entered now let her figure rest out herself. I'd even stop lamping with mates if they got a dog on slip. Or at very least as soon as you see the dog running catch. Walk away from your mate and his dog. And rather than expecting the dog to retrieve, just turn an slowly walk away once he has caught, try getting him to come to heel