in reply to Helping your former company

... but I've been threatened with a law suit once before when I left a startup because I wouldn't come into the office to help out, ...

Unless there was something legally enforceable in your contract/employment agreement that requires your services after you've severed a business relationship, that threat was bluster. Threatening to sue someone is easy.

... and my friend who is a professional sysadmin reiterated several times that I should never ever login to someone elses server without written permission.

Your friend gave you very good advice.

... but also know people are probably saying "This guy is just trying to screw us. Why doesn't he just help us out?".

You know this? Or are you just imagining it? Making sure that you're in the clear legally is hardly screwing someone. If it was that important to them, they'd have gotten you something in writing the first time you asked.