But how about just for the sake of common courtesy, ease of tension, politeness, and the fact that you'll be in close proximity to this person for an extended amount of time? Even though the seat taker is in the right, it can still be done in a decent fashion.
And FWIW, DH and I had booked our flights separately last trip. Booked on the same day, a few hours apart. We both purchased EBCI, but ended up about 10 positions apart. We both purchased EBCI and played by all the rules, yet still ended up boarding separately. My point is that things happen and there's a million reasons why people, who may be doing right, don't end up boarding together. So it's not safe to assume everyone in this position is skirting the rules (and I don't necessarily mean OP's post, just in general).