- isValid: A player who lost the game leaves it and can't be a valid target of spells or abilities.

forge-game/src/main/java/forge/game/player/Player.java

@@ -1999,6 +1999,11 @@ public class Player extends GameEntity implements Comparable<Player> {
 
     @Override
     public final boolean isValid(final String restriction, final Player sourceController, final Card source, SpellAbility spellAbility) {
+        // if the current player has already lost the game, he can't be a valid target
+        if (hasLost()) {
+            return false;
+        }
+
         final String[] incR = restriction.split("\\.", 2);
 
         if (incR[0].equals("Opponent")) {