Some things, one of which your not going to like, the other offers some hope - slight.
First, what you're not going to like.
- The next thing is she wears clothes that cut her crotch front and back, for which she gets sent home from school.
- Her boyfriend is all about sex. Her music about sex. Her clothes about sex.
- She was laying on the couch watching Maury w/ a sports bra and men's boxer shorts so thin and white I could see the color of her skin through them and they were rolled down to her pubic bone. Way down. Who the hell is that for? To irritate me?
To irritate me? Hell yeah, is it working? We rarely give kids credit for much fashion sense (largely because they don't have any) but they are really good at rebellion, making a scene/statement, flouting convention and just generally trying to piss off adults. If she has the body for it, go for it - God knows you don't get that many years for it. And sure the clothes are about sex, and so is the music. By the way, so was yours, you just don't see it. All you're doing by getting so uptight about the fashion deal is proving to her what she already thinks she knows, which that you're kinda out of it. And I'm sure that given the circumstances there is also a very high degree of "Hey, look what I can do" (that you can't) and rubbing your face in it. Is perhaps there a degree of jealousy going on here?
And of course her BF is all about sex. Exactly what 27 year old guy is dating a 17 year old girl for any other reason? In this case, its obviously not her maturity. Although given this, my lover of 6 years is in his 50s. I'm in my 30s, you two don't exactly have any room to talk to them about it.
Now, on to the real important stuff.
One. Blood is thicker than water. Or it should be. Your BF has a moral and ethical obligation to her that up to this point he has totally failed at. His obligation is to her, not to you. If I had even been close to a position where I had to choose between my kids and a GF (and I never have been) I'd toss my GF under the bus so fast she's never hear the wheels turning. Girlfriends are a dime a dozen, but he has this one kid he's trying to do the right thing for. Easy choice for a mature rational adult to make. Morally and ethically, its the only acceptable choice.
Now, it is obvious in this that your BF doesn't have a clue as to how to parent. As any parent will tell you - and popular culture is so full of this that's it become it's own cliche - teenagers, even the best ones, are almost impossible to parent. And that's with some 12-13 years to get used to being a parent, and trying very hard to do right by them so they don't get into this stuff when they hit that age. To have it dumped on you, at the tail end, without having any input along the way, without having some ability - if only through familiarity - with having your voice heard by them, I can't imagine anything more difficult.
He needs to be able to get through to her, to have a relationship with her - and I bet she wants that too - and your just in the way of that to her. I've never seen a 17 year old girl who does not have daddy wrapped around her finger, that's where and how they learn it.
Personally, I really don't think anyone under 21 needs to even drive, much less have their own car. (Yes, I'm an old foggy, and you kids get off my lawn while your at it, and if you would have even suggested such a thing to me when I was 16/17 I'd a skinned you alive. But...) And, at that, their first car ought to be a beater, like most kids first cars are, not something that costs over 10 grand. Deal I made with my kids was, I'll pay half of the cost of the car, no matter what, if you earn the other half, and I'll insure it under my policy which will save you a huge amount of money. But you will pay half, and pay your own insurance. My second kid to this day has not ever owned a car, finding much better things to do with his money. I saw that as teaching responsibility, and choices.
he worries she'll eventually turn to porn or stripping since she is so focused on her body rather than anything else in her life Most kids at that age are very focused on their body, its all they really have that's theirs when you think about it. But, if she does not start doing something she could end up doing that as there are not a lot of jobs out there for people with her skill set. Given that, he ought to help do anything, and everything to change that possible future. As far as I'm concerned fathers of daughters have only one real imperative in life and its to keep the girl off the damn pole.
Some of this is also very petty I think. Like:
Recently her dad gave her a credit card for emergencies only, but she'd managed to wrack up $500 in expenses for nothing (fast food, gas station, cigarettes--yes, she smokes) in about 2 weeks.
Exactly what kind of emergencies does a 17 year old girl have? She might get stuck halfway across the country and need a plane ticket home? Food, gas, smokes - that's an emergency to a 17 year old, it ain't 'nothing' to her, that's you talking there. You're just awful damn lucky she didn't have a clothing emergency, she could have done $500 in a couple hours at Forever 21. As it is, that's what, an average of about $40 bucks a day, not outrageous.
And what kind of idiot (sorry, but that's how I see it) would give a credit card to a 17 year old in the first place? I'd give her a debit card, and put money in every week or whatever, but when its gone, poof, its gone.
You need to step back and give them space. Its not your problem that you created, and you can do nothing to solve it. Sometimes our past just up and bites us in the ass, and that's what's happening to him now.