Sharing Golf Clubs- What are The Rules.

Also yesterday, the fourball in front of us asked for some advice after the second hole. One of their group had forgotten his putter (???) and had asked to borrow one of the other partners. Unwittingly he had given him his putter, and let him use it on the first hole.

Why this wasn't picked up before he used the putter, no-one knows, but anyway after the deed was done they decided the rules had been breached. They wanted to know if we knew the ruling.
None of us could come up with a definite answer, and nobody had a rule book to hand.

So, anybody got the definitive rule to help us out. We had a couple of suggestions, either disqualification or a two shot penalty for each hole that was breached.

But what really had us stumped was whether the penalty for the rule breach applied to the golfer who had borrowed the club, or whether it applied to both players.

Anybody out there who can come up with the definitive answer?