function volunteer_role( $update_data, $user_id ) {

    if ( isset( $update_data['role'] ) && 'volunteer_both' == $update_data['role'] ) {
        $update_data['role'] = 'Volunteer';
    }

    return $update_data;

}

add_filter( 'wpf_user_register', 'volunteer_role', 10, 2 );
add_filter( 'wpf_user_update', 'volunteer_role', 10, 2 );