Can't even tell if this is a joke or not, but either way it's hilarious

Forget getting this guy to admit to the girl that he started the rumor. It really doesn't matter. The trust is broken, it can't be rebuilt by some half assed apology from your 'friend'. All you can really do is apologize to her for not doing everything you could to stop it before it got out of hand. Do your best to communicate your regrets and feelings toward her but respect her wishes if she still tells you to get lost. Even if she doesn't want to talk to you anymore, you can still do the right thing and do your best to correct anyone who believes the rumor. I'd also recommend cutting that 'friend' out of your life.