Kinda depends on the situation really.

If it put the lives of millions of people in immediate danger than yes I would betray my friend. There would be no point to saving one person and sacrificing a greater number of people in most situations.

If it were for something that was already done and held no imminent threat or justifiable threat to the greater number than I guess I would betray country.

I would not directly contribute to his betrayal of country but simply keep my mouth shut if any questions came my way.