There are 2 options to notify the author of a course or a lesson when a user leaves a comment.

Option 1: 

You can use the BB forums in LearnDash lessons, and then the comments show up in the Activity Feed. To insert a discussion, use the shortcode [bbp-single-topic id=28951] and replace the ID as needed.

Option 2:

You can use the wpDiscuz plugin.

Note: If the plugin is not listed on our integrations page, we cannot guarantee that a third-party plugin will work well with our BuddyBoss Theme as we haven’t tried it extensively and also we don’t have an integration nor a built-in function with such a feature yet. You are free to try it, and we are happy to know how it works on your end.