Everything that can be corroded, is corroded even just from the things in the picture. He should be paying YOU to take the bike off his hands. It is worth nowhere near $100, even if you do decide you want to fix it up - again does not make economical sense.
There most likely will be rusted out freehubs, bottom brackets, headset bearings. Basicaly, everything. AND that wasn't a particularly nice bike to begin with anyways. It's sora. That's a $700 MSRP bike when it was new, FFS.