I'm just curios what you think he should have done?
If I were in his situation, I would've kept spitting the facts and be as detailed as possible how I've contributed in that particular role at Twitter. Knowing Elon and his tendency to want extreme detail down to the minutes, I would think this would work in my favour.
I would also explain the situation of having sold the company to Twitter and contractually requested payout in salary, as I understand is the case with Halli. If this were a concern to me and I wasn't being paid out, I would raise that with him.
But at the end of the day, I wouldn't put emotion into it as it can derail very quickly, especially over text messages. I would've also requested a call to resolve the matter offline, if everything stated up to that point isn't sufficient to move forward.
Once the matter is resolved amicably, just drop it and move on.
Personally, I wouldn't want to work for Elon so I would move on to the next venture.
Edit: That's just me and my experience in difficult situations like this. I don't slander, be bitter or vindictive. I remain professional, get to an amicable solution and if there were misunderstandings, clear the air and move forward or if the other party is just foul then drop and move on, never work with them again, never recommend etc. And to be honest this has served me quite well, I'm peaceful and not filled with hate.