When your friend has a son who hangs out with you independently. (aka comes for a visit, occasionally seen at the same bar, etc) He's not your nephew, he's not your friend, he's your frephew.
Most commonly used when the friend is much older and has a son who is around your age.
Person: "Did you go to college with that guy?" You: "No, he's my frephew, I used to work with his dad."