Rule 810.9c

If an effect sets a single player’s life total to a specific number, the player gains or loses the necessary amount of life to end up with the new total. The team’s life total is adjusted by the amount of life that player gained or lost. Example: In a Two-Headed Giant game, a player on a team that has 25 life is targeted by an ability that reads, “Target player’s life total becomes 10.” That player’s life total is considered to be 25, so that player loses 15 life. The team winds up at 10 life.