Dogs who are well trained dont forget it when they are with someone else.My rescue dog was nicely trained before i got her and is the same for me as she was for the fosterer, so why would she be different when she came home from training school?
2 completely different situations.
Ops dog has lived with her and got used to getting away with poor behaviour. He self rewards. He ignores op, be rewarding himself by having fun. That reward is bigger than anything OP has offered him, which is why he continues to do this. This has gone on for years.
The training will be only 4 weeks. Then he will return home where, he will remember that ignoring commands means he has fun.
Your dog was nicely trained for more than 4 weeks and you likely, kept it up. Rewarding good behaviour. That dog likely didnt have years of self rewarding. It had months/years of training.
A 4 week stay away wint make the dog remember what fun he had when ignoring OP.
Your situation = dog spent a decent amount of time being trained, rehomed and kept the same behaviour
Ops situation = dog spent a lot of time ignoring commands and having fun, 4 weeks training, back into the previous environment where he runs circles round OP and does what he wants.
4 weeks in the scheme of things isnt long for training.
Dogs definitely behave differently for different people. And in different environments. Often rescue dogs do act differently to how they were in their home.
As I said, I don't think this is necessarily a bad idea. What I do think is a bad idea, is that it doesnt sound like OP, will be involved in the training. I would have expected her to have to go regularly and be involved in training.
I absolutely understand why she is wanting to do this. I have cockers and foster all sorts of spaniels. Older untrained sprockers can be a nightmare. My latest foster was a 7 month old sprocker who had no training and was absolutely crazy. Wonderful but crazy. We did lots if training with him over 4 months. He went to his forever home recently and they came on training sessions with me and him, so could continue it. He is a fab dog but one of the hardest fosters I have had. And 4 weeks after I got him, if i had stopped training, stopped training him to walk to heel or stay, he would have reverted to his old behaviour