Because it is his first visit, unfortunately he won't be able to use the machines to complete immigration / customs. So:
1) Patience is a virtue. Just smile and don't get cross with anyone.
2) It's not complicated really.
3) Follow the signs and directions of ground staff, you can't easily go wrong.
4) Have with you a print out of your itinerary and the hotel booking relevant for the first night in the USA.
5) Don't take any food into the USA (well, some stuff is OK, but for the first visit best take nothing - no apples, no bananas).

Your friend needs to fill in the customs form on the aircraft, so keep a (ideally black) pen with you. If you are going through any of the Galleries lounges in the near future you can pick up a form for him in advance, there is normally a small stack of them on all the customer service desks in LHR, T3 and T5.

Then the basic process is - queue up at the the foreign vistors' line for Immigration, this can take 30 minutes. In due course, present passport and form and discuss entry with the agent. Your photo and fingerprints will be taken. The CBP officials vary from nice to gruff, don't take any of that personally. Don't be surprised if the agent asks questions that seem a bit odd (e.g. "my son is going to your college, what did you study there?"). For a first visit it wouldn't be surprising if they asked to see the itinerary, see the hotel booking and ask you what you will be doing in the USA quite specifically. When you hear various stamps being placed on to your form and passport you'll know this process is nearly over.

You'll get the form back with a strange mark on it. Then follow the signs to collect your baggage, hand over the marked form to the Customs examination officer. Typically they don't check bags but it can happen. Then you leave that area and end up in the land of the free.