Sit right down and write your skinflint friend a letter asking him to contact you within ten days to arrange to pay off the loan. Use no uncertain terms to state that if he fails to do so, you will sue in small claims court. It is fine to address it to his workplace, since that is the only place you know to find him. But take care to mark the envelope personal and confidential, so that no one else will open it.

For guidance in writing this letter, see Everybody's Guide to Small Claims Court (Nolo).

If your penurious pal fails to cooperate, file a small claims case. If you can prove that he borrowed the money, your chances of winning are good. And if you win, you can turn the judgment over to the sheriff and have the money collected directly from your dear friend's paycheck.