I have to disagree with the people who said make sure she knows she will be going it alone. Yes she probably will be going it alone, but if you say it she will be put in a position of defending the guy and won't see the warning signs herself because she is too busy defending him. She also wouldn't be as likely to come to you because she would be afraid of "I told you so" Keep the act that you like him but be understanding and supportive as she starts complaining about him.
You can also start figuring that you are going to be helping her financially because even if he doesn't skip out, with just a GED his income is going to be so small he won't be able to contribute significantly. For my step daughter the amount of financial assistance she was legally entitled to was so low based on ex-boyfriends income it made everyone's life easier to say fine I won't go after you for child support but you get NO parental rights... he jumped at that chance. It's not what the courts would have done but all parties are happier this way. We are more than happy to help step daughter financially and be very significant in granddaughters life.