Hi, I'm sure this is a very distressing time for you.
There are 2 main points in your situation as it stands. Firstly your boyfriend has no recollection of any criminal events happening. Secondly the female in question has not reported anything to the police.
Unfortunately, it does not seem that your boyfriend will be able to give any clear indication of the actual night and therefore unless any evidence appears then you will be without any further knowledge of the night in question.
If the female has not reported or made any allegation to the police then why is this? Anyone can make an allegation towards anyone else at any time and of any nature. This does not mean that an allegation is true, it does not mean it is untrue.
If it is the female herself saying that rape has taken place and your boyfriend was involved then I would suggest that your boyfriend should seek some sort of legal advice (just in case)
If he can prove (legally) that the crime did not occur then he can have the girl on charges if it can be proven that what she is saying is affecting your boyfriend's reputation and well-being. This is a tricky situation.
I suppose the main point here is that the female has not reported anything to the police. This is your boyfriends argument if anyone tries to suggest he has been involved in something criminal. Unfortunately, we cannot control what people believe or do not believe, that is up to them. Allegations such as these are very damaging and are now commonplace.
It would be difficult to know if he has had sex (consensual or non-consensual) with the female without DNA, witness statements, evidence and specific memories.
The law will not consider whether your boyfriend was too drunk to remember, unfortunately this works against him, as he will not be able to confirm or dispute the events of the night in question. The only help he can have in this situation is PROVABLE evidence.
If the female does not report the allegation to the police then the matter will not become a legal one. If she persists in telling others that this occurred then she can be legally held to account.
I hope this helps and I sympathise in your trauma.
It would be good for you to try and piece together the night in question as much as you can, this will help.
Also just as a reminder, if this situation become a legal one:
IT IS NOT, your boyfriends responsibility to prove his innocence, it is the Crown prosecution service (with the aid of a police investigation) to prove his guilt.
take care, biz.