Why do guys do stuff like this, seriously?

So, my boyfriend has this friend and let’s just call her “A”. She’s a year older than me, and she just kind of urks me. They don’t talk at all and from what I heard, she doesn’t even really like him as a friend, she just feels bad so she talks to him. Her pictures on myspace are of her ass, and her tits. She is attractive, I will give her that. But her pictures make her look like a slut. That’s an understatement. Anyway, I find him sometimes going through her pretty much naked pictures.. Just looking at them, ones he’s even seen already. Tons of times. Why is that? The thing that makes it worse, is that he used to like her, I know I shouldn’t worry over it. But really. It’s just weird.
Another thing is that, she wears clothes that show her boobs, like full on. But if I were to wear a tank top, he’s all “You’re going out like that?”
What’s up with this?