What I find disturbing about Drex is his complete refusal to even acknowledge what a massive breach of trust his actions were.
Even if, hypothetically, he had been friends with the woman's mother for years and only met the daughter when she was an adult, it would still be a breach of trust to sleep with his friend's daughter. Consensual adults? Yes. But in polite society most of us can acknowledge that certain people are off-limits at least until you inform in advance, at the very least, even if you are a free adult who can do anything you like. That's the behaviour of normal people who respect their friends and family. If he were genuinely interested in this woman, the 'right' thing to do would be to approach her mother, his long-time friend, and say 'Hey, I know this is weird but I needed to let you know first. I am really, really interested in your daughter and our friendship matters to me, so I am letting you know I would like to be in a relationship with her.' There need be no mention of BDSM at all.
Instead he hid the whole thing. Whether there was grooming or not the point he misses is that he violated a friend's trust.
Imagine being this woman's mother. Her fight with Drex over her daughter may very well have imploded her relationship with her daughter as well, but even worse, the mother will be thinking for the rest of her life: did he plan this all along? Was he only friends with me to get at my daughter? All their interactions in the past will be reanalysed with this information, any off-colour jokes from him that she dismissed will be met with dawning horror. It is impossible that the mother is NOT going to suspect that he was probably sleeping with her earlier than he claims. For the rest of her life she will be tormented by the thought that she may have exposed her child willingly to a child predator. Whether Drex actually so much as winked at the daughter before she was 18 is irrelevant. Any parent in this position would forever be tormented because they can never actually know the truth.