How much information have you got? Is the friend male or female? (That can make a big difference to your chances of success. Most women still change their surnames if they get married). How unusual is the person's name? (Finding Nelson McTavish could be a great deal easier than finding John Smith).
If you've got a rough idea of where the person might be living, start with the Phone Book:
http://www.thephonebo...ntial/search.publisha
The next step could be to try the electoral roll. If the person hasn't marked the box (on their electoral registration form) to keep their name off the edited roll, it will show up on 192.com:
http://www.thephonebo...ntial/search.publisha
That's a commercial service, so you might have to buy credits (or you could try asking here if a subscriber will take a look for you).
Remember that it's often easiest to find someone who knows the person you're looking for, rather than finding the person directly. For example you might be looking for Jane Pearson who (unknown to you) has now married and changed her name to Jane Muldoon. Looking for 'Jane Pearson' will almost certainly get you nowhere but if you know that her brother is Jeremiah Pearson then finding him could lead you to her.
If you've got an idea where the person might now live (or could have lived fairly recently) a letter to the local newspaper is always worth a try.
Chris