Life is hard to predict. No matter how well you know someone, when money is involved, it’s always wise to have something in writing to record such events, for mutual protection.
Whether you are now borrowing money from someone you know or being asked to lend to one, we would recommend that you use an agreement.
You can easily find templates on the Internet. There are many sites that provide advanced build-your-own-contract services, such as Rocket Lawyer, eform.com, LawDepot, etc.
SoKay provides a more humanized version of these instruments here, which focuses on social responsibilities and potential perils.