If the bet was accepted, it should be paid out in full - according to the odds that were available at that moment.
All bookies heavily protect themselves behind a huge TOC wall which basically allows them to freeze your account, cancel your bets, limit your betting positions - they can do whatever they want.
It is our fault we accept it but it's a good thing we have many to choose from, so when things like these occur you make a mental note and pay attention where you play next time.
This still does not give them any right to do whatever they want and go against their own rules. It's extremely disrespectful, unprofessional, and unethical.
I can counter every single point they make in their T&C about this and it's as if they still wouldn't care or change their decision.
They really need to show respect to their own rules, their users, and fair play at all times.
And I can guarantee you that if the player I bet on lost the match, I would have never brought this up and peacefully moved on just like I always do anytime I lose a bet.
But when stuff like this happens and I'm denied one of the biggest my betting wins ever, it becomes very frustrating.