I ask this because I have on several occasions lawyered up (sometimes adversarially, sometimes just to keep my own ducks in a row) during separations and (if you're working with someone serious who is qualified to manage a founder separation) it's always expensive, but not always valuable.
I also ask this because this is the Nth "I'm splitting up from my startup" I've participated in, and I've noticed that for better or worse, this community has a bias towards dramatically satisfying resolutions, like ensuring for instance that each founder in a doomed startup receives precisely the allocation of company assets that they're entitled to. In the real world, the smart play is often just to walk away.
I've seen almost 10 breakups from startup friends in the last year and have been through one myself. It's like dating — very few end up in marriage and most marriages end up in divorce. Except maybe worse, because people get married after knowing each other for just a few days, in order to apply to YC.
On one hand, if the leaving co-founder retains all equity, it creates a sandbagging situation on a cap table that's no longer useful to the business. On the other hand, it feels right for the leaving co-founder to enjoy some upside for the years they put in.
Opinions will differ here, but I think if you're leaving a pre-PMF startup you've created essentially no durable value, and should return nearly all of your equity.
I've heard of startups doing 10 year vesting for founders (with double trigger) to align this better.