Prevent reset password specific user role

Your problems are

• you’re testing if($user->ID){, i.e. does the b2b_account user object we’ve found have an ID. Which it always will. You probably meant to compare it to $ID.
• you need to return true in the success case, i.e. after your for loop if you didn’t find the user.

However

• you’re ignoring the $allow parameter. You could end with return $allow instead, but it would make more sense to not even do the restricted group check if a previous filter left $allow = false or returned a WP_Error
• it would make more sense to me to fetch the current groups for the user and see if b2b_account is included, rather than fetch all b2b_account users and check against that list of IDs.

So I’d suggest

add_filter( 'allow_password_reset', 'filter_function_name_2698', 10, 2 );
function filter_function_name_2698( $allow, $ID ) {
    if ( ( ! $allow ) || is_wp_error( $allow ) ) {
         // Rejected by a previous filter
         return $allow;
    }

    // Read the user's roles
    $user_data = get_userdata( $ID );
    if ( $user_data ) {
        $user_roles = $user_data->roles;

        if ( $user_roles && in_array( 'b2b_account', $user_roles, true ) ) {
            // b2b_accounts may not reset passwords
            return false;
        }
    }
    // else user doesn't exist

    return true;
}

(using the role check code from here, plus an extra probably-not-necessary null check)