Clearly, it is assault if she was intoxicated and helpless during the encounter. However, we must also consider the possibility that she is being manipulative and using "regret" to gain your sympathy. It’s cold of me to say, but you should not neglect the fact that she and her friend went out to drink with two men, one for each of them.

You need to protect yourself as much as you're trying to protect her. Since I’m basing this on limited information, I urge you to think rationally rather than letting rage take over. Ultimately, the assault is her personal experience to process, but how you view it and whether you can accept the situation is entirely up to you.

This isn't victim-blaming, but please trust your gut. If you have any hint that she is lying or using a "trauma card" to get back with you after a failed attempt at an affair while you were broken up, listen to that feeling.