If he was drugged and sexually assaulted, then he needs to call the police and report the crime. What he claims happened to him is dangerous, and if he doesn’t do something about it, and speak up to the police, other people may be victimized by this woman, too.
I would urge him to do the right thing and report what happened to him to the authorities so that they can investigate and arrest the woman who committed this crime against him, and then press charges.
If he’s not willing to do the right thing, and he still uses this excuse of being drugged for having cheated on you, then I think you have your answer about whether or not he’s telling the truth.
Good luck! 🙂