I started a startup with my best friend whom I knew for several years. The startup succeeded (kind of, we got acqui-hired by BigTech) but the friendship turned into a deep alienation in the process.
Startup pressure can bring out the true character of a person. The problem though is that you may not know the true character of someone until startup pressure is applied -- even if that someone is your best friend.
I'd actually argue for not starting your startup with a good friend. Friendship bias works against you when your cofounder turns against you under pressure. Instead choose a co-founder that you know professionally and that you have a good gut feeling about.
The problem of the story presented is not that he didn't start the company with someone he knew better, it's that apparently no agreements existed that prevented $ANTISOCIAL_COFOUNDER_BEHAVIOR from happening. I'm certainly guilty of that as well. The resources spent on setting up those agreements are always worth it. Also not a shame at all to bring in lawyers at this stage.
It is super common to divide shares generously the first day of the startup, it's like marrying a girl the first day you know her. Isn't it creepy?
It's common to divide 50/50, but with a vesting schedule. If co-founder shows $ANTISOCIAL_COFOUNDER_BEHAVIOR despite agreements made you may kick him/her out and will only lose shares that vested.