What you can do will depend on the state, but nearly all of them have soem recourse. Some states will revoke your driver's license if you don't pay. Some states will garnish your wages. Some will throw your ass in jail if you piss off the judge.
In the end though it's the old blood out of a rock deal. If he lives his life so he's judgment proof,(i.e. he works a low wage job, or gets paid under the table so there's no record of wages, etc.) then it's pretty tough.
Have here go to one of the many women's organizations for this sort of thing. They'll know the real story better than any BS the court will give her, or any thing else.
I have a lady-friend who hasn't recieved squat in years and years. Her ex would just skip and she'd have to track him down, go trough the courts, and he'd pay once or twice, then skip again. Over and over, until he landed in jail for various charges because he was a looser. She'll never get anything while he's in jail, but if he gets out on parole, the court will keep track of him, and make him pay back payments. Odds are though, she'll never see anything, as he'll be back in the system, or dead. Besides, his future money making potential isn't all that great anyway. It's been a long, hard road for her, and I respect her greatly for it. Her daughter is super cool as well, so that just goes to show that if you don't give up on your situation, it will pay off one way or another.
My ex-wife and I are still great friends, and I pay her for my kids without a second thought. I mean after all they are my kids, right? I also see the kids all the time, and we raise them on the same sheet of music. Our divorce has worked out far better than our marrige ever did, and I like her better now that I don't have to put up with any of her crap[:)]. Seriously, our kids had to be tested by the school system, since they came from a "broken home" and were found to be better behaved, better emotionally, and just plain better than kids that come from a supposedly "whole" family. Her parents still like me, and her husband and I are good friends as well (and even his parents have "adopted" me). Sounds crazy, as it goes against all that divorces are supposed to result in, but that's the way it is.
Ross