This is a waiting game really. This dog will not last in this home. They will get fed up and of course blame it on the dog's temperment.
What I would do is let them know that if for whatever reason they do not want their dog at some point that you will help in some way. Be close as they will give this dog up within the next few months I am certain.
If you do not want this dog for yourself, take this time to empower you with options such as rescue groups. So once the shoe falls - you can provide them with great alternatives.