Yeah that is EXACTLY the sort of thing my mother would do, say I didn't have a very obvious injury, and to this day in general conversation she will almost deify my father, telling anyone who'll listen how wonderful he was (which to her he probably was). I know that to some extent she resents my very existance as it besmirches her "perfect" husband, he being only like that with me, not her or my siblings.
I did say vile, inexcuseable things to her, shook her and frightened her, screamed at her for hours until she eventually owned up, so I know what your bf means about being almost afraid of the confrontation for what it might bring about but I got to the point where it really was make or break and my need to know how and why eclipsed the damage that might result. Strangely once we did talk after the initial tears and yelling and finger pointing, she reminded me of some of the nicer things my Dad and I used to do together which I had forgotten ( I was only seven when he died) and it made it far harder to view him as the one dimensional monster I'd developed in my head once I began to recall those things, so in a sense it gave me my father back on far more even terms which I am very glad about.
He needs to have the best relationship with his mum he can have, or he'll end up feeling as I did, that he had somehow been cheated out of a proper family, so I still think if he can steel himself and approach her about it, then it would benefit them both long term, but short term it will make him feel like ****, as it just sort of re-news the sense of dysfunction and rejection.
Have you any idea where his father is now, has he considered tracing him at all, as personally it's my biggest regret that I can't speak to my own father on the subject.
I'm all for confronting and regaining personal power, having tried lots of different strategies, but each to their own, he can only go as far as he is comfortable and that may change significantly over