I'd butt out, although I do think your friend should point out the sex offender laws to his son.
However, I'm astonished at the idea that a 16 year old should find a 13 year old a "little girl". People don't develop (physically or emotionally) on an even timeline. When I was 13, I was routinely mistaken for 18, and there were a fair number of guys who wanted to have sex with me who were a lot older than 16...and had no idea whatsoever that I was only 13. There were girls in my class, some a couple of months older than me, who had no curves yet. I've known 14 year old boys who could be mistaken for 18 year olds (or older), and 18 year old guys who look 13.
Everyone's different. It's entirely possible that the teens in the OP are pretty muich equal, emotionally. The fact that he's sexually interested in a 13 year old doesn't invoke the term "little girl" to me, at all.