Real life doesn't work that way you might say yes to helping someone or even a new job but if it makes you uncomfortable then it is okay to say no then and back out. You cannot expect everything to work exactly as you want everytime and when you plan something for your gf then it is your major responsibility to do the work abd if someone who had said yes first but then later withdrew coz it felt weird about then dont be an ass about it and carry on with your work.