Certainly there is hope in the Small Claims court; however, that hope is somewhat diminished by the lack of documentation (but you already knew that).
The problem will be that the burden of proof will be on your friend. He will have to prove that the money was given and that there was SOME agreement of repayment. If that can be done with the information at hand (emails, receipts, other payments, etc) that is good. If not, then it is obviously a much tougher case to prove, especially if the defendant denies that any money was received at all, or that it was a gift without any repayment.
Your friend might consider bolstering his case by sending the borrower a certified RRR letter detailing the transaction (amount, date, promises to pay, failures of payments, etc.). Then close with something like, "If you do not agree that this money was a loan, or deny actually receiving the money, let me know in writing within "X" days of receipt, otherwise you will be agreeing to all the above statements."
If he fails to respond, it MIGHT help show the court. If he does respond in writing, at least you will know what his position would be in court.
