I have a mod that turns on keepInventory and turns off mobGriefing when you start a world. It works fine, but when my friend(who has vanilla minecraft) tried to join my world he was kicked out with the message 'client brand fml-forge is required'. Why is this happening? Heres my event code:

 

@SubscribeEvent
public void onPlayerLogin(PlayerEvent.PlayerLoggedInEvent event) {
	GameRules gameRules = MinecraftServer.getServer().worldServerForDimension(0).getGameRules();
    	
	if (ConfigHandler.KeepInventory == true) {
		gameRules.setOrCreateGameRule("keepInventory", "true");
		event.player.addChatMessage(new ChatComponentTranslation("[KeepingInventory] " + EnumChatFormatting.BLUE + "Your inventory will be kept on death."));
	} else if (ConfigHandler.KeepInventory == false) {
		gameRules.setOrCreateGameRule("keepInventory", "false");
		event.player.addChatMessage(new ChatComponentTranslation("[KeepingInventory] " + EnumChatFormatting.BLUE + "Your inventory will not be kept on death."));
	}

	if (ConfigHandler.CreeperExplosions == true) {
		gameRules.setOrCreateGameRule("mobGriefing", "true");
		event.player.addChatMessage(new ChatComponentTranslation("[KeepingInventory] " + EnumChatFormatting.BLUE + "Creepers will destroy blocks."));
	} else if (ConfigHandler.CreeperExplosions == false) {
		gameRules.setOrCreateGameRule("mobGriefing", "false");
		event.player.addChatMessage(new ChatComponentTranslation("[KeepingInventory] " + EnumChatFormatting.BLUE + "Creepers will not destroy blocks."));
	}
}

acceptRemoteVersions = "*" in @Mod.