Most Helpful Guy
You stay quiet because even though its your best friend its really non of your business how her relationship plays out.
Lets say you do tell on her. After they break up both of them are going to flip it on you and then you will lose your best friend then tell everyone you know and they will look at you sideways because of it. As much as you want to believe it you aren't going to save the day, you are going to make everything 6 times worse.
The true meaning of a friend is someone who will stick by you no matter the decisions you make even if they are sometimes bad.1THIS IS NOT RELEVANT ANYMORE