Took me 4 weeks after my d&c to stop showing positive on a pregnancy test and another 2 after that to get pregnant. If you are worried contact your doctor and have them check her levels to see if they are rising or falling.
Like Brittny, I also felt pregnant even a month after my m/c. The pg symptoms will only be gone once her period comes again. I'm so sorry.
How long ago did she have a DNC? It takes a while for the HCG hormone to leave the body. I had a DNC because of a missed miscarriage two years ago. I still felt pregnant for a few weeks afterwards. I was very emotional also for a few weeks due to the hormones.
Ask her to take a home pregnancy test. It requires first-morning urine, so get up with her, hold her hand sympathetically, watch her pee on the stick, wait a few seconds, and look at the test together.