Edit - Oh my gods the comments. THE COMMENTS. WHAT HAPPENED TO THE WORLD. This is an article about a 40 year old dude doing the sideways twist with a 16 year old and everyone is all judgemental and angry about it. What the fuck happened?
Well, a few things. In most places 40 on 16 is illegal. Not in Australia from what you're saying, but commentators can be international (like, y'know, us).
Next, there's definitely a betrayal of trust with the friend going on. If it was just "some girl I didn't know, we kept bumping into each other at a coffee shop, then at a party it just happened" the reaction would be different; with the right description the commentators might even call this imaginary waitress a slut.
Third, he's acting like a criminal about it. Acting obviously guilty makes it more likely that people around you will judge you guilty. If he was all like "oh well, accidents happen" and didn't make a big deal of it, the response might be different.
Fourth, 40 on 16? Geez, I'd feel weird dating a 40-year-old, and I'm 27. It's hard to imagine that relationship running much deeper than sex. (And of course, "sex is morally wrong" seems to be a cultural sentiment that pops up from time to time. Damn Puritans).