My roommate ignored our CERN clause and invited his girlfriend—what should I do?

My best friend and roommate was invited to the Large Hadron Collider in Switzerland and can take one other person. I'm a theoretical physicist and I've always wanted to go, I even added a clause in our roommate agreement that says that if one of us gets to go to the Large Hadron Collider they'd invite the other to go with them. However he decided to invite his girlfriend instead for a romantic getaway. She doesn't even like Physics. What should I do?

My roommate ignored our CERN clause and invited his girlfriend—what should I do?
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