All of the above and then some. You could dump oceans in the gaps in my knowledge, and have room left over.
None of that changes that you are recommending a reformat to someone with out knowing the whole story.
if he gave someone access to his computer, you don't need to know anything else - this is a basic rule and you just failed badly.
you can never know if your OS is decent or not, it's probably infected (not sure) and will have registery leftovers which will assist the scammers to reconnect.
i helped many people (in about 5-7 years) on the hardware/software sections, on many forums, i got knowladge and i even know all of the microsoft tasks memorized.
in the moment that someone gained access to your computer, your av and firewall won't help [my friend used to do that without scamming people aka hacking
, and he was showing that to me (btw he stopped to do that long ago!)
anyway, the solution i gave was only a suggestion, after all, im not the one who got scammed - just tried to help him