Let me say this first. You tell us that you do not know for sure that it was not D (you use "IF", "99.9%" and "extremely highly seems like") and if what occurred was a violation of the law or was something wrong being forced upon you (you do not give your age, faith or lack of it, whether you were a drunk or otherwise voluntary participant at the time who later, after sobering up decided you made a bad choice for "having a good time" with a sexual fling the night before and want to blame other's for your bad decisions of getting drunk or whatever and hanging out in a place of immorality, and you say that you reacted but not which way) then BOTH are participants of the crime, BOTH B and D, the doer and the accomplice. Yes, the other, if only one did it the the other was an accomplice if they did nothing to try to stop it. Just like with incest, the mother or father was an accomplice if she or he knew it but to protect their own interests they did nothing to stop it.
Please do not think that you are fine if such a crime was perpetrated upon you and there has been no penitent resolution from the offender and if that has occurred you may not be fine. Sexual sin perpetrates an unimaginable amount of trauma (esp. if the victim was a minor and even worse if it was perpetrated (or allowed) by someone who it is their duty and responsibility to protect the vulnerable child.) A VERY COMMON coping or survival technique in such a crime is for the victim to block out or push far from conscious memory things which actually occurred. I believe it is called dissociative amnesia. The poor child just cannot have in the same matrix of their mind a good daddy or mommy and a terrifying monster. This can happen to someone of any age but more so with children, I believe. I say this because person D could also have been a monster at the time but you may need help remembering it. You REALLY, really, really should see a good counselor.
Sexual sin, sadly today much of such is actually legal today and more such vile acts and perversions are to be made legal, IMO, just as the time immediately preceding the fall of the world empire of Rome. Legal or not if it is sin it WILL be judged by a much higher and powerful JUDGE. Sexual sin is a VERY destructive force and should be stopped and corrected asap.
That having been said, in response to your answer, IF A was 100% positive B is the violator and A desires to be merciful about it then in MY OPINION A should schedule a meeting with B AND D in the professional office of a qualified mediator for such matters and no less. IMO at this stage C is NOT at all a desired element of this resolution; absolutely NOT! You must NOT try to meld the two relationships as you will only destroy yourself. If C is truly a friend they will allow you this space and if they won't they are not a true friend. By what you share how they have already responded I do not think that she is a friend anyway. If a JUST resolution cannot be achieved using a mediator REPORT it immediately and do not put it off. You could be responsible for this monster violating others the longer you put off dealing with this. In MI anyway, if a person is convicted of CSC (criminally sexual conduct) they go on a public awareness site where they are identified as the risk that they are. This crime seems to be an incorrigible behavior for many cases so they NEED to be known where they are so people can keep their children safe.
