My mom put up $500,000 in collateral property to secure a loan for a guy that worked for her. He wanted to buy an old non-functioning granary to turn around and start a business. The terms of his payments were to pay back over 5 years. The loan and whatnot was set up with a lawyer, and my mom was to be advised of all his spending decisions for the granary.
He paid her back in one year, and has been extremely successful since. The granary is still running, and is now also a complete pet and animal supply store, along with a specialty mix store for feed.
My dad put up $250,000 for a local guy to clean up the soil and tanks and replace the pumps for his gas station. No lawyer was involved. This was 10 years ago. He's still trying to pay back because he got the pumps in and then decided with fluxing gas prices that he didn't want to sell gas. My dad has to drive out and bother him a couple times a month to make sure he gets paid, and who knows when he will finally pay the loan back. My parents now could really use the money, but the guy just doesn't get around to paying them, and doesn't really make enough now TO pay them.
Even if they are bestest friends, get a lawyer involved. The guy my mom loaned to was her "almost" son, and was like a brother to me. He spent more time with my family than he did his. Still, they did it right, with a lawyer, and it turned out well.