Here's a quick rundown:
1) You need to have an account at some domain registrar (the same as his, or anyone else).
2) Look through the help/FAQ section on your registrar on how to transfer a domain to their service
3) Initiate the process, where you may need an "authorization code", which your friend can get from his registrar, to approve the process
4) If the domain (at his registrar) is not set as "locked" (or some other restriction), the domain name has been at his registrar for at least 60 days, if it's not just about to expire, and maybe some other rules), then he should receive an email that someone wants to transfer the domain away from him.
5) He just has to follow whatever process his registrar requests and shortly the domain will be in your hands