I had that the other day during the discussions in the MLS thread.
I think it is some sort of double-whammy of betrayal/abandonment for Eva. First her parents, then Des. Much like if you make certain choices for Viv, Des gets a double-whammy, first with Mike, then Viv.
Then there is the question why Des doesn't seem to realise she has hurt Eva. I mean, she may not be the most perceptive person, but I don't think she is that oblivious to her surroundings. And definitely not cold enough to just don't care about her best friend.
A different sort of scenario, doesn't feel entirely right though, but posting it anyway.
Eva is really down because of what went down with her parents. Can't find Des anywhere for some reason. Goes out on the town to get mind-numbingly drunk. While drunk, Des's boyfriend starts to hit on Eva. Eva is too drunk to realise who it is (or she might never have met the boyfriend in the first place). Eva and boyfriend ends up in bed and eventually discovered by Des. Des doesn't give Eva a chance to explain. And Eva can't remember most of it anyway.