Trying to peg down the exact day he first talked to her is really just splitting hairs. Maybe he broke the law by a few months, maybe not, but the problem isn't her legal status, but more her status in life. He started dating her before she had a chance to learn any better. So even if she knows in her heart something is wrong, she has nothing to compare it to. When he tells her that she can't get a dog, or she needs to get her sick ass out of bed and cook his dinner, or that she's
forbidden from going to the hospital when she thinks her panic attacks are physically hurting her, she has no baseline for knowing that's not how a relationship is supposed to be.
If he treated her great, and they admitted they got together when she was 17, it'd just be this awkward footnote from their history, and everyone would laugh uncomfortably and move on. But from every glimpse we see, he treats her the exact way that law is there to prevent in the first place. He treats her like he's a gross old man taking advantage of a naive younger girl.
Unless she finally became wise enough to dump him, in which case