> Over the weekend... He asked to take the conversation off Slack (moved to Whatsapp) and asked if they could hang out (she said, "sure as friends in work context"), referred to her as a milf (ugh...), and asked if he could tell her a secret (she refused)[0]
In the context of the original post, I believe this is one of those cases. To tie this into the analogy to which you're responding, if one of my employees asked another to move off work-chat so he/she could assail him/her and was acting genuinely sleazy, I'd pack up the offending employee's desk, lock down accounts and access on Sunday night, and make sure I was there first on Monday to escort him/her out of the building.
The language that my team uses would make a drunk sailor blush, but I think this is a pretty clear case of justified firing. It's damn hard to hire a great team and nothing will destroy it faster than interpersonal issues.