geezus that thing sounds like a POS, sounds like your friend paid too much for it. I honestly wouldnt know where to begin, it sounds like it could be any number of things...I would certainly start by doing a compression test (which he should have done before he even bought it) that will give you a good idea how good of shape the internals are....if the compression test is very weak. Then I say fuck all the other issues you have because he needs a new motor anyway, then you can go from there. you may luck out and have it be the sensors (a good haynes manual or GOOGLE BITCH will tell you step by step how to test your sensors)

and and ohmmeter will help you and you will need the manual OR THE MOTHERFUCKING INTERNET to tell you the proper ohm range for OEM specs. I flat out refuse to find it for you

good luck