EG. Celebi used Calm-mind, Cofargius used Nasty-plot, Celebi used Psych up. Would Celebi get 3 S. Attack boosts and 1 S.Def boost?
Vrs.
Psych Up copies both the positive and negative changes to the target's stats and replicates them onto the user. The target's Attack, Defense, Special Attack, Special Defense, Speed, Accuracy and Evasion are copied. Any changes made to the target's stats after Psych Up is used will not affect the user's stats.
-Bulbapedia
So no you will keep your stat changes that you gained after using psych up. So Celebi would get 3 Sp.atk boosts and 1 Sp.def boost and kill cofagrigus.