Both of the previous posts provided very good info.
There is no "secret" to being a session player. Normally, it starts when you work in studios as part of a band. The engineer or producers or other players like your playing and recommend you for other things.
So the main key to getting session work - is to play with as many people as possible. Open jams, freelance gigging, free studio sessions, etc. etc. - it's all about establishing a network. Having business cards to hand out to everyone is a key to getting (and keeping your name out there).
I also call most of my contacts every few weeks to see whats up. Every once in a while I get lucky and call someone just as they are starting to look and I get an "inside shot".
Session players end up on the A list or the B list - the A list people get called first. Obviously the more A lists you get on the more calls you get. What does it take to get on the A list?
1) Be nice, be friendly, be accomodating. You can get just as many (if not more) gigs because people like you
2) Be able to groove (vs. having hot chops). Most recording requires simple parts that are in the pocket and that compliment the song.
3) Know many (if not every) musical styles. At least know them well enough to lay down the proper groove (hell, there are a dozen different latin styles alone).
4) Be able to read charts and drum notation. While there are many recording gigs that don't require reading chops - there are many that do.
5) Be able to communicate musical ideas and be able to understand or interpret what someone wants. Not everyone can explain that they want an 1/8 note triplet feel.
6) Be able to nail your parts in only a few takes - commercial studio time is very costly (in particular if there are other paid players, producers, etc.)
7. Be professional - Always!!! I've gotten more work because I'm professional, dependable and easy to work with - then I would ever get simply from my chops.
Candidly, if you are asking these questions for your friend - maybe he doesn't have his act together enough to be looking for session work. If he does have his act together - he should already have this information.