I'm not entirely sure, but I think they would have to do a D&C. Sorry I couldn't be of much help!!
At 20 weeks, she's far enough along she should not have a m/c technically. She'll most likely deliver the baby... In most states (I think almost all states) by 20 weeks you're required to bury the child. So they would deliver it.
By 20 weeks they no longer use the term miscarriage it becomes a stillborn and they have you deliver the baby like Crystal125 said. However, it doesn't happen very often.
Yeah I did forget to mention that. By 20 weeks the chances of that happening are very very low. Usually that only happens if she has something wrong (and if she does she would most likely know by this point) or a placenta abruption, which mainly only happens if she were to fall down, get hit in the stomach, or smoking/drug use... things like that. If she's having a perfectly healthy pregnancy so far, she should not worry about having stillborn at 20 weeks :)
I had miscarried a twin at 3 months, and the 2nd baby was growing healthy. I had my 5 month check up and was informed I was having a healthy boy, and 2 weeks later, went into labor and my baby and he was delivered still born. I think I was 23 weeks, but it is possible my situation is different since I had miscarried his twin during the pregnacy. If your pregnacy is normal and you are that far along, you should be ok! I wish you the best of luck!