Nequid:
For example, see the last post from math where she yet again raises questions and concerns that she would want answers to, as if this has never occurred to OP, and as if OP has not already discussed all of this with her child, and as if OP hasn't repeatedly said that she has.
The problem with 'repeatedly discussing' all I have mentioned with the DD is that teenage girls are frequently willing to paint themselves into a corner in order to not lose face by revealing what has really been going on. This is sadly especially likely to be the case if a teen really has been explicitly pressured or even coerced (date-raped), or if she has something she feels she needs to hide like sexting if that has been used against her.
In the face of repeated remarks on the topic of pressure she may not admit there was pressure, admit she may not have been fully on board with what the BF wanted, admit they didn't use condoms every single time, admit (if this is the case, and I strongly suspect it is) that the sexual relationship has been going on since she was 13, etc. She was reluctant to go to her mother about this to begin with and as with virtually everybody else I have ever known who has been caught red handed, so to speak, I suspect a plausible and reassuring explanation was given -- consistent condom use, relatively brief duration of sexual relationship, intention to go and sort out the Pill.
Plus you have the problem that in the eyes of a young teen 'pressure' to have sex can frequently be misinterpreted as Luv. A young teenager does not have enough life experience to understand what pressure is. Pressure along the lines of 'but I love you and you say you love me' or 'if you really loved me as much as I love you you would have sex' or 'why are you so uptight - don't you believe/love/trust me?' isn't always easy for a young teen to decipher or see as pressure. A young teen might confuse pressure as outright coercion. A young teen might confuse consent on the first occasion with being swept away, the BF moving very fast and her lack of saying No way as consent. Or she might explain away the BF going further than she had consented as evidence of extreme passion for her on his part, and inability to stop himself because she was so irresistible. Plus a young teen might be very susceptible to the physical attentions of a BF who is always all over her, or constantly sending texts (don't know if phones have been checked). There is pressure there, some breaking down of reluctance that may well be consensual or enjoyable but the next obvious step is not necessarily a sexual relationship when you are 14 (or perhaps 13).
Rogue:
'with some teenagers the best way for us to steer them into making good choices is probably not to try to [ ] instil our values into them, but to appear to be very non-judgmental and unconditionally loving in the hope that they have enough self-esteem to make good choices for themselves. Some teenagers will knowingly make poor choices for the sake of make of their own choice. That's rebelling and becoming independent after all. Many an adult has consciously rejected at least some of their parents dearly held values.'
The only quibble I have with that is I would insert the word 'explicitly' or 'repeatedly' in the [space]. I would also be inclined to substitute 'self respect' for 'self esteem.'
Any 'discussion' has to be extremely subtle or you get into the demand-->withdraw dynamic and the risk of rebellion.
Lincoln, you do know your DD best, but you also seem to know the BF pretty well, and so I would like to know more about his family -- so far the picture you have painted has been pretty bleak, but I am wondering about details of the poor relationship he has with his father: what exactly makes it a poor relationship? Has he seen a healthy model of how relationships work in the relationship of his father and step mother? Has there ever been abuse of any kind in the home? What exactly is meant by 'not being in a place where he can parent' in relation to the father, and how long has he been incapable of being an effective parent?
Would you say your DD may be aware of any problems in the BF's home life, and would you suspect that she is in any way or on any level trying to make things better for him -- meeting the needs of others, as Duplodon mentioned? Would you suspect that the BF has flattered her that she can deliver some much needed feeling of being loved by having sex with him? Has she been cast in the role of healer here?