The client wanted to do the following:

• Set the default group navigation tab to Discussions, only if the group has a forum connected to it.
• Set the default group navigation to the selected option on the customizer only if the group doesn't have a forum connected to it.
  

By default, we do not have this option. You can use the following function code. Please see the steps below:

Note: Make sure to have a complete site backup before proceeding

    1. Go to Appearance - Theme Editor.

    2. Under Select theme to edit, choose (BuddyBoss Child/whatever theme is active), then click Select.

    3. Below Theme Files, select Theme Functions (functions.php).

    4. Append the code just before the closing PHP tag “?>”:

add_filter( 'bp_groups_default_extension','bb_custom_group_forum_default_tab');
function bb_custom_group_forum_default_tab( $default_tab ){

  $group = groups_get_current_group();//get the current group

  if( empty( $group ) ) {
    return $default_tab;  
  }

  // Make sure it has a forum attached to it.
  if ( bbp_get_group_forum_ids( $group->id ) ) {
    return 'forum';
  }

  return $default_tab;
}

    5. Click Update File to save the changes.

Note: Any modifications are considered as custom work already. Know more about our Support Policy here: https://www.buddyboss.com/support-policy/

Screenshots

Without Code (by default, the default tab is the Members tab):

With Code: