Well, I'm assuming that you're talking about how to connect the amp to the deck. If you also are not sure on how to run the wires from the front of the car to the trunk I suggest you get some help by somebody who's done this before as getting the power wire through the firewall is a pain. Most people I talk to run their power wire down one side of the car and their RCA's down the other side. You said he would be using the stock radio which I don't believe has preamp outs (what you would connect the RCA's to) so you have a few options. Ideally your radio would have sub out preamps. If not You can buy a line level converter which you attach to you speaker wires, usually the rear, which will then give you a preamp output for RCA cables. Some amps have speaker level inputs which is basically a line level converter within the amp. These are usually not the best quality and is a step below buying a line level converter. I do not know if the 250D has speaker level inputs which is sad b/c I have the 500D, your friend has a nice amp.
I have no idea what you know about installing systems so if anything isn't clear just ask and i'll do my best to elaborate.