OP, when the owners come back please be honest about how it went. They should know honestly so that in the future they can help the pup get over its challenges.
On a funny note, a friend of mine got a shi-poo and mentioned a few times that she was having trouble potty training it and was ready to get rid of her little pup. It was her first dog. Another friend and I gave her some help, but told her she needed to contact a proper trainer because it seemed like the really needed to start back at the beginning and build a foundation (e.g., the dog refused to pee outside, only inside on rugs, and she was trying to train the dog to pee on the toilet...for real). She got a little help and said things were better, and she wasn't going to give up the dog after all.
Fast forward six or eight months, she called needing a dog sitter for the next day for a family vacation after her sitter fell through. I sent her to my dog trainer, someone very experienced who works with high-risk rescue dogs and someone who has really helped our boys develop nicely. She took the dog, and when she came by the following day to walk our dogs, she told me the following:
-the dog is only fed veal, almonds and fruits and vegetables, and will not eat from a bowl, only fingers
-the dog also came with candy treats (real sugar, real multi-coloured Skittles-like candy)
-the dog was 80% unpotty-trained
-the dog was sooooo happy playing with other dogs, but it looked like she had never met another dog at all. No manners, no idea how to hang with other dogs.
-the dog was significantly overweight
-the dog had no idea of the leash and was scared of it.
My trainer took her on and did the best she could - and asked the owner what changes she would like. My pragmatic trainer friend said she would only work on the things the owner wanted worked on, because if the owner didn't follow up, there was no point. Except the candy, she threw that out. So she worked on socializing and potty training.
My friend called me in a panic because she asked for a progress picture of the pup and was worried because the dog was grubby. I assured her it was because the dog was getting exercise outside and that my dogs came home from the trainer often grubby and tired, and they often went in the creek, but I promised her I would check up. Indeed, her little pup loved going in the creek with the other dogs and was having a ball being a dog amongst dogs, but I quietly told my trainer friend to make sure the dog was clean when she was picked up. LOL. When my friend went to pick up her pup after 3 weeks, she was so upset that the dog was running around with other dogs and didn't make much fuss that she was there. And the dog had dirty paws. And she was a little lighter. No mention of the dog being much more consistently potty trained, having fun being a dog, her strange feeding routine was honoured, and having been safely kept in a home environment. My point was the my friend was so out of touch with what the dog needed, and loved her little dress-up pet. TO each their own, I guess.