Thanks for asking this question.
As the clients buy from theme authors, not from us, they should receive updates from the theme authors. Clients don't have the license key, and they can't have automatic updates.
Theme authors are responsible to update the extensions as they bundle or integrate extensions within their themes.
When someone find a bug or a security problem, they should contact the theme author and ask for updates immediately.
This works similarly for other plugins (such as Revolution Slider or WP Backery PageBuilder) that the theme authors integrated inside their themes.