OP - It depends on exactly HOW drunk she was.
If she was really drunk, but still able to move around freely, speak to others, etc, all in all, being DRUNK but not utterly HAMMERED, then no, I don't think that it is a reasonable excuse for cheating.
However, if she was so drunk that she was incapacitated (and I don't mean unconscious - the law sees these as two separate issues - you cannot give consent when UNCONSCIOUS and you also cannot give consent if you are too drunk. Two. Separate. Things.), then she was a victim of sexual assault.
But none of us can know which category your partner falls into.
There are some very worryingly 1950 esque views on consent on this thread, which make for very sad reading. Still very much in the vein of "it's up to women to police their behaviour otherwise what do they expect"? Having worked with victims of rape before, they often blame themselves, even when there is (quite obviously) absolutely no need for them to do so. How wonderful, therefore, that there apparently (still) exist people who are willing to attribute blame in the same way.
It's really quite pathetic really.