@MrDamage... Re: One Invoice is all it takes
Dude! Lets get back to reality.
First, doing what you suggested would trigger a lot of potential legal headaches.
The guy is off the hook.
He could easily have not been the nice guy and had not offered to provide any additional... unbilled support. He is within his right to tell the company to stop bothering him.
More to the point, he's no longer under any contract therefore had he accessed the systems, provided any information which may have yielded any downtime due to a person following his advice, he would have opened himself up to criminal prosecution as well as civil liabilities.
Without seeing the contract, the clause that the previous employer relied upon to harass the guy would either be invalid / unenforceable, or enforceable under limited circumstances. (This is going to be true depending on the jurisdiction.... laws differ by state, country ... YMMV)
If this were in the UK, he could easily tell the company that he's not going to do anymore free support in that he is now working for another company. They could sue him, however, loser pays and in this instance, the company would lose with the case getting dismissed w prejudice.
In the US, the contract was terminated, there is no implied warranty and since they have since hired multiple staffers, any implied warranty would have been voided months earlier.
A competent lawyer would have a field day.
To your point, he could have easily told the former employer that he was out of a contract and that they could renegotiate a new contract as soon as he completes the work for his existing customer... and then set a high rate, along with other reasonable terms which the company may find distasteful. (e.g. like an upfront retainer.. plus payment at the time service is rendered. ) The moment they refuse ... then they are no longer a problem.