In this particular reply, all the 4 things you mentioned are NOT happening in the story. (At least by what we know.)
Unwelcome: We don't know if it was unwelcome. She even agreed to move to another communication medium (one which wasn't officially work related.)
Overt: It was on their own time, on a completely distinct communication medium. I don't see how more COVERT it could be.
Repeated: Again, from what we know, this was on a single weekend. He called her a milf, asked if he could tell her a secret and she refused. That's it. If he's not asking again and again, it's not repeated.
Workplace sexual advances: That's not in the workplace, nor on the company's time.
There's not much we can conclude without knowing more.