Virtus's Maneuver

You may call a player a friend or a foe even if that player will be instructed to perform an impossible action. For example, for Regna’s Sanction, a foe may control no untapped creatures.

Players can’t take actions in between the time that friends perform their actions and the time that foes perform theirs. If any abilities trigger from friends performing actions, those triggers won’t be put onto the stack until the foes perform their actions and the spell finishes resolving.

The designation of friend or foe is only relevant to the spell that asks you to choose. A player you call your friend doesn’t become your teammate, and the next “friend or foe” spell you cast could name that player your foe.