Earlier quoted context omitted.
I get the whole leaving on good terms, really. But this whole "hey, we just want you to keep the code and answer questions for our other engineers" aspect really ought to have set off red flags. Pro-tip, hand write a nice letter to your boss/colleagues on your way out. Bingo, no burned bridges, and no risk of law suits for holding on to company IP.
Yeah, I hear you, hindsight 20-20. It was Mark's manager John Shimmin who made that request. John is a great guy, and Mark had (still has?) a great relationship with him. He genuinely wanted to help Shimmin, and did so faithfully for weeks. It's not fair to pin this on Mark and dismiss it with a "rookie move dude." Mark (and I) had no idea that facts could be twisted and ignored to create this kind of legal conflict.…
It's a sad lesson and an emotional mistake. You didn't owe the employer/managers anything and should have cut all ties to the company especially since these projects are somewhat related.
You are both good guys for wanting to help out and it sucks that things got twisted up. I hope things work out well and I wish you all the best.