The question wasn't asking HOW his dna or other identifying marks ended up on her car but rather, why. Perhaps I missed it on the various documentaries surrounding this man but I didn't see where she was taking her car to him for repairs or maintenance. She was apprehensive going to his place for her job but did she also go to him as a mechanic?
I'm not sure it's on record or not, I think you'd have to be heavily tied into it and not dependent upon a documentary. Regardless, the amount of times i've helped people at work on their cars when I'm not an advertised mechanic, or even professionally a mechanic, is staggering. I just know cars, so when someone goes "I have a weird noise, think you can listen to it?" I say sure, walk out, open the hood and give it a listen.
His DNA on a hood latch shouldn't be admissible as it's completely irrelevant to the context of the case. Touching a persons hood latch or vehicle shouldn't implicate you for murder, much less make you a suspect. I guarantee there's 20 people's worth of DNA under the hood of that car from sweat.