"outsmarting" a DNA test

i'll keep it as short as possible.

not long after my husband and i started dating, he told me that a girl who he hated dated briefly was pregnant and said it was his baby. he told me he was very doubtful that the baby was his. he went through chemo and radiation as a young teen, and doctors told him the chances of him having a child were slim to none, he also told me the dates didn't add up. although he had his doubts, throughout her pregnancy he attempted to go to appointments and be supportive. she wouldn't let him come and would only ask for money and send him ultrasound pics. 

well fast forward to baby girl being born. i told him to go ahead and get a DNA test because since they had sex pregnancy is always possible. we paid extra for a blood test because he felt that was more definitive than the standard swab testing. results came back that he was not the baby's father.

a few weeks ago she messaged him on facebook saying she heard we were having a baby, and how it must be a miracle baby since supposedly he couldn't have kids. she doesn't know he took fertility drugs to help, not that it's her business. but anyway, she says she wants another DNA test because she said that he "outsmarted" her with the first one and that's how he got the results that he wasn't the father.

now she's posting in social media that he's a deadbeat and not taking care of his child and how karma is going to handle him. even her mother is posting these things!

i feel like this chick is crazy.. how do you go about "outsmarting" a DNA test, and a blood one at that!?