offering 15%. Treat your friends as friends, not as an investment vehicle. If your parent's friends are within 5 years of the end of the mortgage, they likely have multiple options to pay it off other than your parents.

If your parents are wealthy and financially stable (doesn't sound like it) and have extra money laying around that they don't mind losing, then it's OK to lend to the friends.

If it is money they need, and are evaluating different investment options, then the answer is: No. Don't do it.

If this is money they are counting on in retirement, the answer is: NO, ABSOLUTELY NOT, DON'T DO IT, RED FLAG, FULL STOP.

Best option is a date targeted retirement fund, such as the vanguard one listed above. With a 10 year horizon, that fund will be managed conservatively, to lessen the risk of loss of principle.