What should I do after I almost hooked up with a guy that my good friend really likes?

You have two options here: Tell your friend or don't. Sometimes it's better to keep some things a secret. I'm sure it will hurt your friend deeply if you tell her, and it might ruin your friendship. If you're willing to risk that and "do the right thing" go for it. If you're not going to tell her, maybe confront the guy and make sure that your little "secret" doesn't get out, because if she finds out from somebody else, things will be even worse.