So what do you suggest if someone refuses to come along? Knock them out first? Sedate them? What he and other passengers think is irrelevant, if a captain decides you’re not going, you’re not going. End of story
The captain is allways allowed indeed to decide it's a no-go. But the captain then has to respect legislation: tell the passenger he is denied boarding, give him the indemnity (400 € for a Faro-Stansted), rebook them on the next possible flight, and eventually book a hotel.
When crew (cockpit or cabin) refers to unruly behavior, they must respect the Contract of Carriage. And Ryanair's C of C is clear: when safety or security is as stake, passengers must obey all orders. But for all other matters, the C of C states that the crew must be reasonable: "...if, in our reasonable opinion, we may take any measures we consider reasonably necessary to deal with the situation, including restraining you. You may be removed from the plane and refused a seat on any other flight, and may be prosecuted for offences you committed on the plane...
That someone asks to be seated next to family member, is no unreasonable question. That someone switches to an empty seat next to a family member, is not unreasonable: it happens with all airlines regularly. How this cabin crew then managed that situation, is uncertain. But the newspapers quote witnesses, saying that the passenger wasn't unruly or abusive. So there must have been a better way to solve the issue then calling the police.
When a passenger is removed for disruptive behaviour than no indemnity needs to be paid, thankfully! That would be the world upside down.
Passengers tend to side with a difficult passenger once the decision is made to remove him from the flight (unless he was a pain in the ass to everyone). Why? Because people tend to defend the (what they perceive as) underdog.
It has become normal these days that people treat people in customer facing roles like pieces of shit. They are impolite, obnoxious, intrusive and claim to have a lot of rights that they don’t actually have.
The C and C is clear. In OUR reasonable opinion. Not the passengers, not the family, not the other passengers, no the airline and so by appointed directive ... the captain. If in his opinion it is needed to remove that pax from the flight, then that’s how to cookie will crumble. It’s up to a judge later on to decide whether or not it was not reasonable should it come to that.