I play Intermediate men`s fast pitch. My teammate wants to wear a "one ear" batting helmet, is this still legal? What about the half helmet for a catcher / first base ?
To find the answer to your questions you look at Rule 1-37 in definitions. I assume this individual is talking about fast pitch.
Rule 1-37 says a batting helmet must have two earflaps. (Rule 1-37a)
The catcher helmet may be the scull type without earflaps. (Rule 1-37 d)
A helmet used by any defensive player does not require earflaps but must be same colour as the team hats. Rule 1-37e and Rule 3-6f-2 and f-2a.
The definition of the helmet in Rule 1-37 is required for all offensive players. A catcher must wear a helmet but Rule 1-37d allows the skull cap without earflaps because the catchers mask has ear protection. Note a hockey goalie type mask is legal for the catcher.