You can install any packet sniffer software to capture all the packets sent on your network. You need to then filter the packets based on port number, say Yahoo Messager uses port 5050 (for example), then you need to decode all the packets sent on port 5050.

This way you would get all the messenger packets sent on your network. Once decoded you would be able to find the persons friend list and all messages he sends and receives.

However this works with messengers that wont encrypt data, for example MSN Messenger encyrpts the messages so the above wont be of any help.

If you need any further clarificaiton please get back to me.