You make this choice for yourself as well as each other player. In some rare cases, you may wish to call yourself (or your teammate in a Two-Headed Giant game) a foe. You can do that.

Friends perform their specified actions before foes. This means that a friend’s action may cause a triggered ability of a foe’s permanent to trigger before that foe’s action causes that permanent to leave the battlefield.

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.

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.

If a friend has zero cards in hand, that player discards nothing and draws one card.

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.