Lost Helmet by Defender?

[golfdesigner]

With the new rule regarding a helmet lost during play requiring a player to sit a down if the helmet coming off was not the result of a foul. That is easy to figure out. What I haven't seen is what if B1 loses his helmet, then continues to participate in the play, like making or assisting on the tackle or by defending a pass. Would that be illegal participation? or do you have nothing other than to send him to the sideline for a play?

Rule references would be helpful.

Thanks