By "dating her" he surely means "having sex with her", right? I think it's wrong just for that. People should stop using each other. It's selfish.
The other reason, pretty much has to do with selfishness as well: she is 16. She is a girl. Maybe not physically, but it is most probable that, mentally, she is still a girl, and will continue to be one for 8 to 10 more years (sometimes for her entire life). It takes a lot of time and experience in life to become a real adult. And the truth is, most of the people who call themselves "adults", are still mentally teenagers. So let us say, this 28-year-old friend of yours is thinking like a teenager. Typically, using the "tail", when he should be using the head.
So if your friend is intelligent enough, he will keep his "thingy" in his pants.