Ironic things happen in a game – don’t they! I’ve seen a hit ball rifle up a fielder’s arm and into his glove sleeve – out of play but the ball is still live and in play. I’ve seen the ball get stuck in the webbing of a third baseman’s glove and the ball is still live and in play. I’ve seen a pitcher get a bazooka shot to the groin and actually stagger back right to the second baseman -collapse in pain and lock his legs shut, while the second baseman dug it out and the ball was live and still in play.
As long as the helmet was in fair territory, and not deliberately entered into the path of the ball so as to deflect or otherwise alter the play intentionally, and the ball remains in fair territory – I would play it as a live ball unless in the umpire’s judgment there was obvious belligerency to fair play – in which the umpire under ORB# 9.01(c) may enforce a judgment appropriate to the situation. Protests by either club can be field appropriately if there is a disagreement to that umpire’s decision.