This was most likely a case of handing over a wrong boarding pass to someone who then didn't think much of it and just boarded the wrong flight.

If this person had been so devious as to somehow obtain the OPs boarding pass, surely they would quickly realise the flaw in their plan when the original pax attempted to sit on his lap.

I wonder if many here would so quickly jump to accusations of "identity theft" etc if the other pax had been described as an average white male business person-type. I can't help but think if that had been the case, some would much easily explain this as a simple mistake, end of story.