It is not that we are all perfect and have never made any mistakes. It is that when we have screwed up we accept the consequences and then get on with things, particularly where the matter is legal or related to a contract which we have agreed to.
Very recently I exceeded the checked baggage weight limit for a different airline by a couple of kgs. I had to pay an excess baggage fee. Irritating, yes. Did I start a Facebook campaign, seek someone else to blame, enage the media, demand an apology, fish for compensation.... no I didn't.
O'Leary is right on this one, although his way of speaking about it was not ideal. There is virtually nowhere on Earth where in the space of two weeks you cannot print off some pages from the interent; and presumably she managed to print her boarding passes for the outbound sector so cannot claim ignorance for the inbound sector.