Isn't the answer to all of this "did I do the wrong thing to my friend" business that,

1) You should have helped him have his shot at the girl until it was clear it wasn't happening, and then,

2) Respectfully requested that you have your turn with her

Now that it's said and done, I do think you need to apologize for not informing him first. I don't think you needed to ask permission, just had a conversation with him about whether his chance was blown so you could both agree whether it's your move now. I mean, why is it that you didn't shoot him a text the moment you thought something might go down?

Honestly, with the housemates... I lived with 3 girls once. I'd have an empathetic yet stern talk with them about how the girl thinks you spilled about hooking up, and have them tell the girl that it isn't what happened. What reason does the girl have to believe "They made it up"? She won't believe you. Fix it at the source. If you appeal to them right, they'll find some way of letting her know that you kept quiet while also saving face for themselves. They'll probably just tell her that they were misinformed by someone and thought that you already told people. Hell, they'll probably tell you that because they're red-handed and scared. This is not your fuck-up, not your clean-up. If you just get mad at them, they might not help you. Appeal. Incentive. If they're concerned that you might let people know that they were lying - especially the people that they first said it to - they'll put the effort in to fix it. Bring their shit back on them and they'll undo their shit. Make it work... but for the love of god, do not beg. Do not get involved past that. If they don't help right away, move on, forget it all, and try to keep your friend.