I know someone who left the country while on parole and he had also overstayed a student visa for nearly 20 years. He claims he doesn't know whether or not he was banned from the country. He's afraid to find out and he's afraid to come back to the states. Is there any luck of him being admitted back into the country? If so, will he just be arrested when/if he gets here?
It's actually very simple - your friend will require a visa to enter the US, so they should apply for one, and be sure to declare their previous issues.
If their visa is approved, then they will most likely not have any further issues when entering the US.
If their visa is rejected, then they will not be allowed even travel to the US (unless they are planning to enter via Canada/Mexico - in which case they will not be allowed enter without a visa).
Based on what you've said it would seem unlikely they will be given a visa, but it's impossibly to say for sure without knowing the full details - applying is the only option that will give a definitive answer.
It's actually fairly easy (although not quick) to find out. You can file a freedom of information act request with ICE and find out what they have on file for you (you can't request the information of someone else). The correct agency should have something on file indicating whether or not there's a ban and what the duration is. This should also indicate whether or not it's possible to file a waiver since it tells you what kind the ban is (it should show a paragraph of the immigration act).
Edit: I think you need to have an address in the US for them to send the information to. So have your friend send a filled out and signed request to someone in the US who can mail it to ICE and receive the information.