I've tried to read most posts, but don't think anyone has mentioned 'transference'.
It is essentially a psychological process by which we transfer longings, desires, and emotions we had for our parents onto other figures who hold a similar authority over us (they may have formal authority like teacher/ priest/ doctor, or it may be that they are much older than us, or have some kind of 'status' that makes them feel more powerful or wise than we are). It can be very powerful and it's one of the reasons why teacher/ priest/doctor/Therapist positions tend to forbid sexual contact, as the transference effect can make anybody regress to the status of a damaged child.
None of this is an excuse- but if she is incredibly drawn to your DH, she is probably, as some have said, quite damaged. Most specifically, she probably has a father who left her feeling unsafe or unloved or unseen, and she has transferred the longing for that onto your DH.... probably all unconsciously. So suddenly she has met a man who seems to stir something in her deeply, who seems to offer the attention and nurturing she missed, who is safe, older, wiser, unknowable, who is invested in our development, seems to be proud of us and excited by us.... like we often want our fathers to be.
For your husband it can be a powerful cocktail.... just at a time when he's feeling his masculine life is over, when he's almost invisible to people, when he's losing or lost his potency, he has a young woman looking to him for everything, engaging with him erotically, 'bringing him back to life', acting like he's a god, longing for him...,, who he feels protective towards, because he senses the vulnerability, and if he's a basically good man, will be telling himself he is saviour, rather than exploiter....
As I say, none of it excuses, but this may be what's happening on an unconscious level......
I'm sorry you are going through this....