Puppy housetraining issues

Hi there,
I am posting this on behalf of a work colleague.
Last month she adopted a 5 month old GSD. He is doing really well but his toilet training is a bit to be desired!

When someone is home with him ( which is a majority of the time) he sits nicely by the back door and asks to go out, he is crate trained, she leaves him for no more than two hours at a time. When she gets in he has messed in his crate. I advised her to get a smaller crate as the one she was using was very large. She did this and he just flicks it out of his crate instead She has tried just leaving him with the crate door open, and he just goes on the kitchen floor.
She does have another GSD, and they get on really well. She is really at a loss as to what to do next, and has asked me to investigate. I am going to suggest she gives him a stuffed frozen kong to see if that helps, any advice would be greatly appreciated. Thank you

Beck xx