Hust to play devils advocate, so if a guy is drunk at a party, goes into a dark room, where he fucks a girl thinking it is his girlfriend, would that girlfriend believe in a honest mistake if she caught them red handed?
This has nothing to do with what Fred believes, or what this girlfriend would believe in your more extreme scenario. It has to do with intent and context. Fred was with her, entered a room beside her, but she went in first so she didn't see which room he went in. She saw a dick pop into the hole from the room next to her, she was drunk and knew her husband just went into a room. She assumed it was his and started sucking it, thinking he was wanting to have some fun. When she realized her mistake she immediately stops and apologizes.
In your hypothetical, the only context you give is that a guy is at a party, goes into a dark room and fucks a girl he thinks is his girlfriend.
Was he AT the party with his girlfriend? I assume yes but you never gave that context
Did him and his girlfriend agree to meet upstairs to have sex? You never said.
Did the girl look anything remotely similar to his girlfriend? Again, you didn't say.
If the answer to any of these is no, then of course it's cheating and not okay, but that's a completely different and way more extreme scenario. If the answer to all of those is no, then god damn, there's no getting out of that one, that relationship is almost certainly over.
If the answer to all of those is yes, then sure, it's possible that it's an honest mistake. At the same time though, it's a more extreme example, and they were in the same room with each other. Whereas Mary saw a dick plop through a hole, which she thought was her husbands.