Fiance accused of cheating but claims he does not remember

My fiancé of 5 years and a few of our friends went out one night. There was alcohol and drugs involved. My friend stayed at our place on the couch. She contacted me the next night saying my partner came into the lounge and had sex with her. When I first spoke to him about it he was like "no way" and that he would not do something like that, ever. She is mentally unstable however I still don't know if she is capable of lying about something like this. She is now pursuing him in court for rape charges. These are the facts I know. - He remembers approaching me at the club when I was chatting to some guys as he was a bit annoyed I am flirty when I am drinking - He remembers being in the cab on the way home - He claims he has no memory of getting home - He remembers waking up in the morning and having a shower and vomiting (this was approx 5 hours after getting home) - There is a 5 hour period he claims he has no memory of - There was lube all over the couch (my friend took it home with her as "evidence") - His cousin who took the same drug (mdma I believe it was) said he couldn't remember having sex with a girl he went home with - The lube is located in our bed side table, therefore he would have had to get the lube and I would have been sleeping in bed I am struggling dealing with this and coming to terms this could have happened. We had a very healthy relationship and all his friends were in shock as he is not this type of person to do something like this. We are engaged and have a house together. I was a priority to him for everything, he was probably the best boyfriend I ever had. My issue is... could he be lying he doesn't remember as he is scared of the consequences? Or could he actually not remember and didn't know what he was doing at the time? Could he have thought it was me? I just don't understand how he could of been in another room to begin with rather than in bed with me. The girl claims she "tried to get him off multiple times but he wouldn't, she would only wake up as it was happening and he kept coming back" I am considering getting him to do a lie detector test as the DNA results won't be for a few months. I have already been dealing with this for 3 months and I can't handle it anymore. I need answers. Any advice appreciated. Thank you

SUSIEDQQ has excellent advice. He needs representation, and you need to withdraw from any interaction with his accuser. As for your relationship: Do you think you'll ever be able to trust him again? Whether it was consentual or not, he made a choice to cheat. He then fenced himself in with a straight denial. Once he realized that wasn't going to work, the only place to go after that is to claim he wasn't in control at the time. Even if it was because of the drugs, he chose to take them. Even if what he's saying now is true, that doesn't make him a good guy. It makes him a guy that hurt you, and your friend. Even if he was so high he wasn't thinking straight, you'd think seeing you alone in the bed you share, while he was getting the lube to use on someone else, would have brought him around enough to rethink it. Too much is not adding up. I have no idea if anything anyone is claiming is true. It will be the job of the courts to decide what happened, based on evidence. If it were me, I'd already have enough evidence to know I shouldn't hitch my life to his. But that's me. What's your gut telling you?