If she didn't send the tweet, he wouldn't have got fired. If he didn't get fired, he wouldn't have posted on HN about being fired. If he didn't post on HN then none of this would've happened.
Basically it all stems from her original tweet..
I'm not condoning any of the behaviour but it could've quite easily been handled more maturely. If, instead of turning round, taking a photo and tweeting she'd approached him like an adult (maybe with the conference organisers if she was scared) and said "That's not cool" to him so he could actually apologised to her and none of this would've happened.