If you take his box and card and plug it into your dish then it will work.
If you took his card and used it in your box then it will not.
Is he still in his first years contract with Sky?
If so, the box is meant to stay connected to his phone line for the first year. May cause a problem if its left disconnected for too long but one month shouldn't be a big problem.
Just don't plug his box into your phone line!
As for breach of Sky's T&C i don't know, but at a guess I would say you are not allowed to move the box to another house. But the real question is, will they find out?
I know of people that use Sky on the mainland Europe without any problems!