She could tell from his sample that his sperm count was low, that the motility of his sperm was sub par. Fortunately, he was practically infertile. She knew these things having worked in a fertility clinic prior to forensics.
She could also tell that he had not committed the rape. His DNA did not match.
She knew that he had beaten her sister, and gotten away with it, that he'd beaten another woman so bad she was blinded.
She had no compunction about falsifying his report as a "Positive Match" at risk to her job.
It was worth the risk.