exactly why you don't do favors for people you don't know. Just make sure it's in perfect working order before he gets it, and tell him "everything works fine, if you have any questions I suggest you contact the manufacturers of the parts or search for information on google because free time is hard to come by for me. (Casually mention how you barely found the time to build this PC)
Then, if they ever ask you for help, don't let them tell you anything about it and tell them you will get back to them.... but don't.
They should have listened to you in the first place. Actually, come to think of it, they should have figured out that if you wanted to do this all the time, you'd have started a business. But if they need help with something as simple as computers, then I think they'd have trouble to begin with.