The emotional status of the coworker is going to be in one way if he lost the promotion amidst other competing colleagues excluding you; it will be altogether different if he lost the promotion by competing against you and you were the one to get the coveted post. So, the ways of helping the coworker to come out of his bruised feelings will have to be different.
If the co-worker did not get promotion and you were unconnected with that, it is far easier to offer helpful counsel to bring him back to normalcy. The process will be facilitated further if you happen to be a good friend of him, whereby he can freely confide his emotions to you and also be willing to take your counsel.
If the colleague was truly meritorious and deserving to get the promotion, but failed on account of other reasons, you can always highlight his positive strengths and re-assure him that merit will never go unnoticed and it will not go unrewarded. You can convince him saying perhaps the time is not ripe for him to get his due reward or perhaps he is destined to get something bigger and better.
As a way of consoling him, you can also point out the pitfalls associated with any promotion – like added burden of responsibilities, possibility of getting transferred to an undesirable locations, working and dealing with too demanding and unreasonable bosses, difficulty of managing more man-power, possible loss of mental peace and the possible demand of more time to be spent at the office etc. Anyone looking for a promotion would not have probably thought seriously of all these matters. At a time when not getting the promotion looks to be a great loss, these “alternative points of view” can really help one to think from a different angle and feel relieved for a while.
If the person who did not get the promotion lost on account of competing with a better qualified and truly meritorious person, then the counseling has to be done in a different way. Indirectly, but not hurting the lost co-worker’s ego, this reality may be conveyed in subtle ways. Words like “You see, sometimes it is not just hard work but it is smart work that gets rewarded; you should know how to work smarter than before; and the bosses should also come to know that you are smart enough to hold higher responsibilities; just observe how successful people work and learn from them”.
If the coworker lost the promotion in competition with you, the last thing you should project is that you are more meritorious and deserving than him. If you want your relationship to continue well and if the person is going to be your subordinate hereafter, then you should be smart enough to win his heart, not by bossing around, but by continuing to treat him as a colleague and a worthy team mate.
Accompanying him to the canteen, meeting him casually at week ends or going to movies together (if you had been doing it earlier) may have to be continued. Sometimes, complaining to him about the stresses and pains associated with the new responsibilities may even help him to feel relieved that he didn’t entangle himself into them fortunately. By your talk, body gestures or official interactions, you should try to maintain a posture wherein your co-worker will not feel that he is belittled just because he lost in competition with you.
Good interpersonal skills are needed in handling such emotional issues. Those who are gifted with those skills will evolve to become good man-managers in the future.