The risk is that the business fails. There are no "sure thing" investments, especially at higher returns.
Ask him how his business can fail. If he doesn't give you a variety of possibilities and ways he's hoping to prevent them, I'd be very worried that he's overconfident and not prepared for difficulties.
A friend offer me a investment opportunity at 15% annually, is it too good to be true ?
He was my coworker. I know him at work for years. It is unlikely he take money and run away....