What do I do? My girlfriend's bother fancies himself as a Makeover designer?

no way, a good designer listens to his clients. You were clear that you wanted the fireplace to stay, and no bright colors! that is your style, and he should of gone with a different color palette. He should of came to you with a design board showing the colors, patterns, and vision for the place. He obviously had his own vision for the place, that was his taste, if he cannot design for other people and not himself, then he won't get very far in that profession. I'm mean come on, what was he thinking, he did exactly what you told him not to, did he think that he was going to make a design so fabulous that you would love it? it does not work that way, if he listened to you, talked with you, asked for samples of things that you like, styles you like, colors choices in other parts of the apartment.. listening to the client, and giving them what they want, is a good designer, he needs to give you those receipts and chalk it up to lesson learned.