Create/Update CPT title from relationship title

I have 2 CPT's:

• Lead
• Activity

Here's my code to create the relationship:

add_action( 'mb_relationships_init', function () {
    MB_Relationships_API::register( [
        'id'   => 'lead_to_activities',
        'from' => [
            'object_type' => 'post',
            'post_type'   => 'lead',
            'meta_box'    => [
                'title' => 'Activities',
            ],
        ],
        'to'   => [
            'object_type' => 'post',
            'post_type'   => 'activity',
            'meta_box'    => [
                'title' => 'Lead',
            ],
        ],
    ] );
} );

When I create a new Activity or update an existing Activity, I want to update the post title/name/slug using values from the custom fields and relationship. I've figured out how to create/update the post title using the custom field values, but just can't figure out how to get the post title from the relationship.

Here's my create/update title code so far:

function iswp_update_activity_title( $data ) {
    if( $data['post_type'] == 'activity' ) { // Check post type
        if( isset( $_POST['activity_date'] ) ) {
            $title = $_POST['activity_date'] . " | " . $_POST['activity_action'] . " | " . $lead;
            $slug = sanitize_title_with_dashes( $title,'','save' );
            $slugsan = sanitize_title( $slug );
            $data['post_title'] = $title ; //Updates the post title to your new title.
            $data['post_name'] = $slugsan; //Updates the post name aka slug to sanitized version of title.
        }
    }
    return $data; // Returns the modified data.
}
add_filter( 'wp_insert_post_data' , 'iswp_update_activity_title');

I just can't figure out what I'm supposed to do to get the $lead to actually return the post title from the related Lead.