bp_activity_update_mention_count_for_user( int $user_id, int $activity_id, string $action = 'add' )

function bp_activity_update_mention_count_for_user( $user_id, $activity_id, $action = 'add' ) {

	if ( empty( $user_id ) || empty( $activity_id ) ) {
		return false;
	}

	// Adjust the mention list and count for the member.
	$new_mention_count = (int) bp_get_user_meta( $user_id, 'bp_new_mention_count', true );
	$new_mentions      =       bp_get_user_meta( $user_id, 'bp_new_mentions',      true );

	// Make sure new mentions is an array.
	if ( empty( $new_mentions ) ) {
		$new_mentions = array();
	}

	switch ( $action ) {
		case 'delete' :
			$key = array_search( $activity_id, $new_mentions );

			if ( $key !== false ) {
				unset( $new_mentions[$key] );
			}

			break;

		case 'add' :
		default :
			if ( !in_array( $activity_id, $new_mentions ) ) {
				$new_mentions[] = (int) $activity_id;
			}

			break;
	}

	// Get an updated mention count.
	$new_mention_count = count( $new_mentions );

	// Resave the user_meta.
	bp_update_user_meta( $user_id, 'bp_new_mention_count', $new_mention_count );
	bp_update_user_meta( $user_id, 'bp_new_mentions',      $new_mentions );

	return true;
}