Just to go against the grain here. Sometimes, loaning money to friends is the right thing to do. For example, they had a loved one die, and need the money to cover funeral expenses until the life insurance pays out. Here, you may consider it your ethical duty to loan the money (or you may not). It does not make sense from a financial point of view (you are unlikely to charge interest and you are taking all the risk), but sometimes you put your financial prudence aside because "being a good person and a great friend" is more important. It is true that the general rule should be not to loan money to friends, and particularly never loan money you cannot afford to lose. But there are exceptions to every rule.
I'll note that you may be best off with a plain-english, non-lawyery contract. Make it absolutely as simple as possible. As others have pointed out, specify a repayment date or schedule. Note what happens if they miss the deadline. Specify interest, if appropriate. Get it signed by you and the other person. And make sure you consider what happens if your friend doesn't honour the agreement.
For that kind of money, it is also worth considering collateral.