bp_activity_adjust_mention_count( int $activity_id, string $action = 'add' )

function bp_activity_adjust_mention_count( $activity_id = 0, $action = 'add' ) {

	// Bail if no activity ID passed.
	if ( empty( $activity_id ) ) {
		return false;
	}

	// Get activity object.
	$activity  = new BP_Activity_Activity( $activity_id );

	// Try to find mentions.
	$usernames = bp_activity_find_mentions( strip_tags( $activity->content ) );

	// Still empty? Stop now.
	if ( empty( $usernames ) ) {
		return false;
	}

	// Increment mention count foreach mentioned user.
	foreach( (array) array_keys( $usernames ) as $user_id ) {
		bp_activity_update_mention_count_for_user( $user_id, $activity_id, $action );
	}
}