Firstly, don't fall into the trap in taking on someone elses 'mistake' your planning your own pup stick to that and don't be swayed, it is most important that you give your planned pup your full attention, one dog at a time. Two pups together could well be a complete disaster and cause heirarchy problems. We all feel terrible for dogs given to silly owners like this, the rescues are full of them, we just can't save them all, you have to be tough and stick to the plans your making.
Secondly, if the dog came from a reputable breeder then the owners just need to pick up the phone and the breeder should give them all the training advice in the world to help them settle a boisterous pup and should be interested in taking back the dog as well. If the breeder is not interested............
Thirdly, do not try to give info yourself, it drags you in and you will feel guilty and end up with the dog, tell them to join CD, we're brilliant

We can also give the owners lots of advice or point the owner in the direction of a good behaviourist, don't get involved yourself.

Fourthy, if they can't be bothered to work with the dog and just wish to 'get rid' then rescue is the answer.