I have a friend who is 26 who is seeing a 16 almost 17 year old Guy. Is this sexual relationship legal because of the difference in age?

The age of consent is 16 in NC. Unless your friend is in a position of authority over the child (teacher, boss, etc.) its probably legal. Though its morally wrong and disgusting.

What does his parents have to say about it?

"Seeing" isn't illegal.

The age difference has nothing to do with it. If one is a minor has everything to do with how legal it is.

How do you know there's a sexual relationship?

You just hear about the sex stuff.. . 2. I don't think his parents mind cause they seem happy but still

If the one is 15, it is illegal, again, sad the parents are OK with it, shows what bad parents they are and this is an example of someone able to take advantage of a younger person.