I agree with Cathpot, it is perfectly plausible that both parties are telling the truth.
I'm going to bow out of this, Dawn, as it is far too close to home for me, but I'd like to give you my advice first if you don't mind.
I think you need to go with your gut instinct. If you don't think your husband is a rapist, honestly, you are probably right. We can all give our opinions but they're not worth anything because we don't know him, or her.
I think it will emerge that your friend has other issues, as well as this one. How you tackle her is up to you. It will take great strength to get past this and rebuild the friendship...and even if you want to, she may not.
Your relationship with your husband is a separate issue. When the trauma is less raw, you need to examine whether there are any trust issues in your own relationship. Whatever else happened, he's admitted getting into bed with another woman, and not telling you about it. I would be considering counselling for both of you in the not too distant future.
Finally, look after yourself. None of this is your fault. None of it. It wouldn't have been avoided if you were a better friend, or a better partner. They are both adults. You are a victim in all of this. Get support where you can, whether from friends/family or a professional.
Good luck. You sound like an incredibly strong person(I'd had a bit of a breakdown by now!)
You are amazingly showing both loyalty to your husband and compassion to your friend...that can't be easy. I hope you can reach an outcome that gives all three of you peace of mind.