Capot, I'm not sitting on the fence. If I had both sides of the story and refused to make a judgement then you could accuse me of fence sitting, etc. I have one side of the story and without the other I simply refuse to make a judgement.
As far as documentation is concerned I think you are probably correct to the extent that people in general fail to distinguish between a boarding pass, a ticket, a confirmation and an itinerary (there are probably more terms/documents but I can't think of more right now).
What you seem to fail to understand is that if you are travelling with an IATA airline (and she almost certainly was) then resolution 722 requires the agent/airline to provide a paper copy of the e-ticket. It very carefully lays out what data items that document shall contain. I don't have the exact wording to hand because I'm currently in the US.
There is always a way of getting hold of a copy of that document before you leave and I strongly recommend you do so. I have recently entered Hong Kong, New Zealand and the USA. Hong Kong and New Zealand didn't ask for my ticket, USA did. Because I had my copy I had no hassle but I'm aware of people who, because they did not have the paper copy were deported from the USA. The story I was told was that the sole reason for refusal of entry was lack of an e-ticket piece of paper - I have no way of knowing the truth of the statement.
In the past I have been asked for tickets in several countries. It used to be easy; to travel you had to have a ticket ergo (unless you lost it) you were able to produce it to immigration. Nowadays with e-tickets problems such as the one we are discussing do occur. Although the airline has to provide you a copy it is your responsibility to make sure you have a copy of the document to present on request/demand.
If you want me to speculate I would guess that the inability of the young lady to produce the e-ticket was simply one item in a chain that caused UKBA to refuse entry.