Sure, he’s 30 and she’s 41.
Which isn’t that crazy in reality, but he’s of the forever-21 face of great Asian genetics and supposedly fantastical-Korean-skincare-products-at-your-disposal variety.
Hyun Jae (Yoon Si Yoon) and Bo Hee (Yoon Son Ha) were in a relationship together, twenty-ish years ago.
And in present day (in the drama), Bo Hee has a son who is the same age as him.
So by way of my completely logical understanding, it’s like he kissed his friend’s mom.