I got an aggressive call from my former manager, and he wants me to tell them how to fix something. The problem is that I don't remember and I no longer work for them, so can he sue me for not helping them? Do I even have any obligation towards my former employer even if I could help? It sounds ridiculous and I've experienced seen something like this before. Assume that this takes place in the U.S. or Canada.
It is not uncommon for an employer to ask a former employee to assist with something as a courtesy, and sometimes the former employee will choose to do so. If it requires more than a small amount of time, this may be done under a short-term consulting contract for pay.
But unless there was a contract of employment requiring such post-employment advice, there is no legal obligation for the former employee to provide such assistance. The most the former employer could do is give a poor reference if asked by potential future employers, and most large corporate employers now only give job title, salary range, and dates of employment to avoid claims of incorrect or defamatory statements in such references.
No (with exceptions and qualifications). It might be wise to try to help and there are narrow circumstances where it might be required.
In general, there is no legal obligation to do so, although it could result in a negative reference from your past employer if you don't, which could be quite economically damaging to you as a practical matter.
Also, in some professions (generally licensed professions), such as law or psychiatry, there may be an ongoing professional ethical obligation to the firm's customer to prevent prejudice to the client/patient related to work you were involved in, even if there is not an ongoing legal obligation to the firm itself. Failing to honor these professional ethical obligations could cause your license to be terminated or suspended. But you are entitled to fair market value compensation from the firm, in most cases, if you provide that help, under a theory called quantum meruit.
It is common for sophisticated and moderately sophisticated employers to require some kinds of post-employment cooperation without charge or at a set rate, in an employment contract, most often with litigation related to intellectual property or other matters (where you might be needed as a witness), or by establishing a consultant contract for a fixed monthly sum of money when terminating a key employee or taking over ownership of a company.
Normally, if you leave a company, the company either decides that your work wasn't needed, or they hire someone who will replace you. If they decide not to replace you, or your replacement isn't up to scratch, that's their problem, not yours. If there are jobs to do where only you knew how to do them, bad luck for your previous employer. It was up to them to make sure that all such information is handed over during your last weeks at work.
The only situation where you might have to help is if you have required passwords in your sole possession, if you signed up for third party services under your name and not the companies name etc. For example, my work laptop is protected with a password that I created - I'd have to give them that password if it takes them a while to figure out that they don't know the password.
Normally your old company would ask politely, and offer you payment for your services; you are then free to help them or not, but should then ask your new company if they are Ok with that.