When the ABS has a problem, the system is supposed to disconnect from the brake system. This way you still have a normal system.
There are speed sensors on each of the front wheels (out near the rotor) and one on the transmission. There is a computer, and the actuators (under the hood, with all the brake lines coming out).
Codes will set, and can be checked. The problem area may show up (like a speed sensor bad), or not be so easy to locate.
At my smog repair shop, we work on brakes and just about everything, except ABS problems. That's because they can be a real bitc* to find, and a lot of $$$ to fix.
If you (and your buddy) can't figure it out at home, you could spend SOME $$ to have it diagnosed by a brake specialist (NOT MIDAS). Then you can decide if you want to spend more to fix it.
Like I said, you will still have a car that the brakes work like a car that didn't come with ABS.:biglaugh: