Just as a general recommendation, not particularly relevant to this case as it happens - since this dd's BF seems rather evasive, quite the opposite of seeking to charm the parents into offering their blessing to the relationship - I would recommend the film 'An Education', starring Carey Mulligan. It's a nice bit of social history, if nothing else.
I was prompted to think about it the other day in the context of a case of what appears to be some level of coersive control, in my wider field of acquaintances, in which the husband does in many ways, have a pair of very intelligent, educated, capable parents, eating out of his hand and deferring to him. It is interesting, if disturbing, to watch, in real life.
That film popped into my head and I realised that it is not only the less educated, unworldy, provincial parent that can place themselves in a position of actively nurturing a coersive boyfriend or husband.
My general point, which might have some relevance here, is that there is a very fine and nuanced line between acceptance and encouragement, between supporting the one you love in making their own choices and endorsing, so cementing, a particular path. Also in choosing not to comment upon disquieting observations, so as not to rock a fragile boat, thus failing to cast light upon events which might later have coalesced to illuminate a pattern and a problem.
I don't envy you OP. I agree that what really matters is whether this individual is kind. I agree with pp that any attempt to prevent your dd from talking openly to you would be deeply concerning. I also think it is very, very important not to allow yourself to be 'persuaded' into saying things you don't believe (and I've just 'got' a reference I saw the other day, on another thread, to dogs and horses).