It sounds to me like he is using a messenger service such as MSN Messenger, AOL Instant Messenger or ICQ.
These allow you to see if any of your friends are online. MSN's uses hotmail addresses whereas you set up accounts specific to the other two (you get a screen name with AIM and a number with ICQ). You then enter the screenname/number/e-mail address of your friends and if they are online you can send messages to each other.
I have accounts with MSN, AIM and ICQ so I use a program called Trillian which lets me access them all through the one program so I don't need them all running at the same time.