Well this is certainly a loaded question, and I am not even going to try and navigate the responses to it. But I will answer the question honestly. Though I suspect my answer will not be indicative of the typical response here because I am coming from this at a very different body and identity politic.
The short answer is: I wouldnt be that position.
The more proper answer goes something along the lines of this.
I'm straight edge. I dont drink or take recreational drugs, I will play a little loose and fast with the general straight edge consensus on casual sex (EG: I actually practice casual sex) I would never fuck a girl when she was shit face drunk, even if we where in a long term relationship. I dont really tow the straight edge party line, I have nothing against people who do drink or take drugs... as long as they can maintain there composure. If your falling over yourself drunk, I just want no part of it, I cant think of many bigger turn offs then some drunk chick.
In a hypothetical situation where some generic guy fucks some generic girl who is drunk (tales been told a thousand times.)well... if he's sober and taking advantage of her drunken state, he's a rapist. The morel quandary of gets more complicated if they are both equally drunk. How can I possibly pass more blame to one party over the other just because of there gender? I cant. My knee jerk reaction is to still blame the guy, but that's not a fair response. If no infidelity is involved... well they're both idiots, and I really find myself more ashamed for them then morally outraged.
Since your example did how ever state she had a partner, then infidelity is involved. I have a strange attitude towards infidelity. I have never cheated on a girl, but I have been cheated on and I have played the roll of the other man. When I was cheated on, I had a hard time caring, I'm just not a man who is prone to jealousy. When I played the roll of the other man, well, I made no promises to her boy friend, no commitment existed between me and him. Managing her relationship is her problem not mine. Of course, when ever we fooled around, she was sober... so there is that.