Offhand, it seems you should tell the boss. I also think it would not be a sin of detraction.
Several things could weigh against this. Your boss may tell the person late to you that you informed on him. Or he could learn another way. Then very well he could become your enemy and you would have trouble working with him from then on–or he could even try to get you in trouble with the boss. If such a possible result seems possible, it might seem better to give a noncommittal answer.
There is also something that strikes me as being undesirable about informing against a colleague. Something similar could happen if a parent insisted his children informed on each other. This could do damage to family life–or the operations against your place of work, to your own detriment.
However, I understand why some would think it necessary to tell your boss about the infraction of your fellow worked. But I don’t think you should do it without taking account of the possible consequences.