She reported it to the founder, presented evidence, and made it clear that the conduct was unwelcome. I'm done at "milf".
She made it clear to the FOUNDER that the conduct was unwelcome. Did she tell it to the new employee? Did he repeatedly talk to her in the same manner after being told it was unwelcome? We don't know.
I have no problem calling a coworker an "asshole", especially on my own time while taking a beer with him. Without context, that the new employee said "milf" doesn't convey anything other than he found the lady sexually attractive. His seducing techniques might be abyssal, or she might be the kind of girl that likes a little spices.
Again, we don't have near enough information to make a judgement here.
"Well, I just figured I'd grope you because you might like it, and it is after 5 pm..."
"Subject: possible sexual intercourse --- Good morning Miss, We met earlier today at Acme corp. I wanted to bring to your attention my desire to meet you in the goal to have intercourse. Please answer before 5pm. You can reach me on my cellphone.
Regards, Mr. X"
The more I think about it, the more I realize that the boss has no responsibility. He might want to intervene if the relationship is influencing their work performance, but that's all.
What happens when you don't fire the guy? Say later on, for whatever reason, SHE has a bone to pick with the company. Now the FOUNDER is the guy who got a harrassment case and "didn't take it seriously". What's the founder's responsibility to protect his company? Don't see how he needs that risk.
In addition, the event took place outside of work. You are not responsible for the social/sexual relationship of your employees in their private life.
Which is why you need to consult an employment lawyer and/or your internal HR expert.