I wasn't in this situation but I know someone who this has happened to. The girlfriend and the boyfriend are both close friends of my boyfriend, and what happened was really *****y. The (now ex) boyfriend is still obviously negatively affected by this - he's become a much more irrational person now.
The girlfriend and my boyfriend were at a friend's party, to which the girlfriend apparently got drunk (and presumably high as well), resulting in her kissing another guy (her ex-boyfriend) at the party. Now, I don't know if this was a quick peck on the lips, or a full-blown make-out, but it was a *****y action regardless; the alcohol doesn't excuse it. Even the guy she kissed had a girlfriend at the time, and I'm pretty sure they've expressed interest in getting back together before, which makes the action seem less of an "oops I was drunk" and more of a subconscious process.
My boyfriend was there when it happened, and you can clearly tell he immediately didn't know what to do. The girlfriend pleaded for him not to tell her boyfriend, insisting that it was a drunken mistake. My boyfriend decides to not tell her boyfriend despite being somewhat close with him, but instead decides to drop hints as he is concerned about the possible mental health of the boyfriend if he were to tell him what happened. This of course makes me upset as I felt it was unfair to hide this from the boyfriend, and he deserved to know the truth, but I somewhat understood that my boyfriend didn't want the guy's mental health to be on the line.
One week goes by and the girlfriend breaks up with her boyfriend, giving no explanation other than "We want different things in life." The boyfriend spirals down to depression, thinking that her lack of explanation meant that something was wrong with him, or that he wasn't enough for her. Dude was devastated. He just didn't understand why their split was so abrupt, and the whole time he was blaming himself and thinking he was worthless. While I do think that at times he said things and acted irrationally, you could tell the vague split got to him and his mind trailed to all places.
Another week passes and the ex-boyfriend finds out - not through his ex-girlfriend, but through a mutual friend who wasn't at the party - that his ex-girlfriend cheated on him at the party. He was absolutely distraught and outraged, he couldn't even piece together his thoughts properly. All this time, she had cheated on him, given a vague explanation for their separation, leading him to believe that he was inadequate and that he lacked a certain element in the relationship.
She didn't even apologise to him. She merely told him that they weren't in a relationship anymore, it didn't matter because she was drunk, and that she didn't regret her choice. Essentially, she called him irrational for his reaction to her cheating.
It doesn't matter if you're drunk or not. It is cheating either way. If you get so drunk that you make questionable decisions when under the influence and are aware of this, then the solution is to drink less or eventually stop drinking altogether. It's easier said than done, but it's not like she's an alcoholic - as far as I know, she only drinks at parties.
That being said, I think it's extremely important to tell your partner when something like this happens. Their reactions may vary depending on their individual differences; some may be willing to continue the relationship knowing it was a drunken mistake and see that you are genuinely remorseful, or some may not want to continue the relationship on the basis that you are responsible for your own actions. Either way, I think a person owes it to their partner to tell them if they have cheated whilst drunk. If they do hide the truth, eventually it will catch up with them, and could be more damaging than telling them upfront. A lot more people would be willing to forgive an honest person than someone who hides the truth.