Please do not take any action that might be construed as threatening or harassing. This could go against you if the matter came to court for whatever reason.
You write that you do not have a great deal of documentation but this can be remedied. At the very least you should be sending regular accounts for the amount outstanding and you should invite your ex friend to make contact if he/she has any kind of objection/excuse for not repaying the debt. After the second account you need to initiate a series of "letters of demand". This site has a form you can use as a guide.
By writing to this person you establish a documentary paper trail. If you are uncertain of the address, you could send letters via a workplace. Certified mail will cost you a few $$$ and you will increase your case that the accounts are reaching the right person.
At some point you will want to start levying reasonable interest charges but you should clearly identify a starting point for these charges. By all means, specify that the interest charge will be backdated to the date of the initial loan.
Putting things in writing is a way to clarify this situation for both parties. It is likely that your ex friend is uncomfortable with the way things have deteriorated. Give this person a chance to "come clean" and acknowledge the debt and make some steps towards repayment. You may find that your friendship is eventually restored and this is infinitely more valuable than the money.
