The game should be played according to the FIE fencing regulations. The fencing mask underarm protector are all essentially protective measures. So not wearing them is at the fencers own risk.
Absence of an inspection mark immediately warrants a red card for the fencers and may even be barred from the bout. For improper dressing equipment the fencers gets a yellow card followed by black card for repeated offence.
If the fencers doesn't have inspection mark on the mask as well as underarm protector he/she may not be allowed to fence but the decision depends upon the referee.