Playing With The Wrong Golf Ball

Golf Nut, I was playing a round of golf with my regular foursome last week when we hit a situation that we didn’t know how the golf rules handle. About halfway through the round, one of our group realized that he was playing the wrong golf ball. It was the exact same brand that he uses, but he normally puts a mark on his ball and the ball he putted with didn’t have that mark on it. He didn’t know when he played the incorrect ball. Is he penalized for this?

The answer to this golf rules question is not if he is penalized or not, because he is. The question is really how much. Let me explain what I mean by that.
Rule 15-3 deals with playing the wrong golf ball. 15-3b deals with what happens if this occurs during stroke play. You didn’t indicate, but I am assuming that you were playing stroke play and not match play. First if he makes a stroke or strokes at a wrong ball, he is penalized two strokes. This rule also states that if the golfer doesn’t correct his mistake before the next tee, he is disqualified. So part of the answer depends on when he started using the wrong golf ball, which it appears that you guys didn’t know.
Decision 15-1/3 deals with this situation, the golfer doesn’t know when he started playing the wrong golf ball. And it clearly says that under Rule 15-3, unless the golfer can verify that he started playing the wrong golf ball on the current hole, he is disqualified. Decision 15-1/3 clarifies some other scenarios around this but the end result is the same. Difficult ruling to say the least.
Now, for me, unless we were in tournament play, obviously I would let him keep playing and probably assess two strokes. But if you guys were in a big golf tournament or one of the competitors decided to enforce the rules, your buddy is fresh out of luck.
Thanks for the question and good golfing.