In this particular case though - a weekend conversation after an explicit request to take it private - not sure if this qualifies as "job related".
And even if it somehow does qualify as being 'job related' I guess my point is that I can see how by agreeing to switch to whatsapp the signal that she sent was 'yes, let's talk in private' and if he's the kind of dude who calls someone he barely knows a 'milf' I can totally see how in his mind this was 'yea let's hook up' :)
When does someone become "more" than a coworker? There's no clear line there, but it's definitely not during the very first private conversation.
Ppl also get harassed on the street with all kinds of inappropriate comments.
The question here is whether this constitutes a workplace sexual harassment or not. It's probably going to be a judgement call on behalf of the employer, however they have to be careful not to get a wrongful termination lawsuit to deal with down the line if they do end up letting him go.
You're right that people get harassed by strangers in all sorts of public places, but that's not what happened here.