In an exported calendar, the HTML tags and entities are not filtered out of the description area (i.e. your Post Content).
To fix this, you can apply the following changes to
Line 69, change
$content = apply_filters( 'the_content', $event->post->post_content );
$content = html_entity_decode( strip_tags(apply_filters( 'the_content', $event->post->post_content )), ENT_QUOTES, 'UTF-8');
With this, you can have a nice cleanly formatted description view in your exported calendar(s).