Solution:
The issue really isn’t those lines in the one plugin; it sounds like the host has capped php memory usage for each account in their global php configs. Ask them.
You can try these settings in wp-config.php, but there’s no guarantee they will override the hosts’ settings:
// memory for each php process
define(‘WP_MEMORY_LIMIT’, ‘128M’);
// memory for WP admin
define(‘WP_MAX_MEMORY_LIMIT’, ‘512M’);