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.

When Virtus’s Maneuver resolves and you choose friend or foe, first the next friend in turn order (or, if it’s a friend’s turn, that friend) chooses a creature card in their graveyard, then each other friend in turn order does the same, then each of those players return those cards to their hands at the same time. Friends will know choices made by earlier players when making their choices. Repeat this process for foes, who choose a creature one at a time and then sacrifice them all at the same time.

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.

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.

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.