I feel you!
I understand perfectly what you are going through, but my advice is to stay out of it.
I know they both are your friends and you love them and care for them, but please don't choose sides and don't tell Bobby about Sarah, either Sarah about planning to tell Bobby what she did. It won't end well and for some reason maybe Bobby doesn't want to find something like that or maybe he won't believe you.
In the end you might lose them both in your way to make things right.
Just take a moment and think.
You said that from the outside their relationship seemed to be going really well, but what if they are not? What if Bobby doesn't make Sarah feel good ? Or what if their act is just too good that they don't realize how much tey are lying? What if Bobby is the one pushing Sarah over the edge? What if Bobby cheated on Sarah too? What if?
There might be a lot of scenarios so don't jump to conclusions.
Since Sarah told you about that, she might felt really bad about that and that secret might be eating her up from inside. And somehow she felt like telling someone her deepest, darkest secret in order to free herself from the guilt. Maybe she tries to deal with the fact that she cheated on a good man.
If I were you, I'd stick with Sarah and make her see the truth before hurting Bobby more than she already did. But not trying to threaten her with: I will tell Bobby everything. Try to highlight to her how much they both mean to you, and that by telling you that secret, she maid it hard for you to look at Bobby. Repeat her how she means to you and that you want her best. Tell her that her secret is safe with you. But in the end Bobby will find out (karma) so she better tell him. Make her realize that her decision wasn't good and that if she misses something in her relationship she should end things before keep hurting Bobby. And try to make her tell you the real reason why she did that. Because there is always a reason behind every act!!!
Threatening someone even as a friend, is never ok. I know your intentions are good and pure, but she won't see it like that. She will see it as an act of betrayal. Be by her side and help her go through all of this. She trusted you with her deepest secret and that means you can't go simply to Bobby. As I said previously, there might be a chance that Bobby won't believe you or that he already knows.
Just trust the Universe and Karma and things will set the way they should. Just don't choose sides before knowing perfectly both sides and even then, you might not know the whole truth.
Take in consideration the fact that even after Bobby finds the truth , he might forgive Sarah, but not you. And in the end you don't want to lose them both.
Ps: Be by Sarah and make her turn herself to Bobby but not by pushing her or threatening her. And be by Bobby's side too (by defeating him in front of Sarah).
Ps2: Make Sarah understand the burden and the huge amount of responsibility she put on your shoulders when telling you the truth. Now, cause of her act, you have to lie and betray a friend in order to cover for her. And if she s a good person, which I vote she is, down there, she will take the quilt out of your shoulders and tell Bobby the truth on her own. And in the end if this scenario happens, you will end with two best friends, not just friends!