Why does a boss?

My friend is not getting deserved promotion. She is t. Mgr and the previous Mgr left making her t. Mgr. She got half way promotion to t. Mgr. She does it all what the Mgr did. The pay is not the same. The previous Mgr did not do aloooooooot of things and she is paying a price for it. The current boss was friends with the previous Mgr. They still are friends. She is ignoring what all the previous mgr did not do. From inside info, the t mgr has learnt the current boss does not intend to promote her. Is it race? She is from Asia. What do bosses do this? Misuse of power? What to do? She has no guts bec she is Asian and feels her life will be made a hell by the current boss who is in power.