How to get a friends dog to spend time alone?

Don't be surprised if you fail spectacularly. Border collies need lots of mental stimulation, plus this dog is not used to being left alone. That's a recipe for instant separation anxiety, a sample of which you experienced in the damage he did to the door. You will probably have to resort to crating him and perhaps sedating him if he hurts himself trying to get out of the crate, unless you are game for taking him to a veterinary behaviorist to get him on Prozac or a similar drug.