My friend cheated on her boyfriend. She feels really badly and is freaking out...how do I be a good friend to her? Should I even bother?

He’s a nice guy, though I’m not particularly close with him aside from seeing him every other weekend with my friend. They’ve been together for about 6 months total, the last month being LDR since he’s moved to a new city about 3 hours away. I thought they were a good couple and never really took her for the type to cheat, but she disclosed to me that last night she went out with coworkers and ended up going home with one of their friends.

I was shocked and disappointed – if my boyfriend cheated on me, it would be OVER – but she was very distraught and in tears. She was very drunk, though she realizes it was her decision and she regrets it deeply. She’s been crying all day and feels terrible, and she is also very emotionally unstable and used to suffer from severe depression (she used to harm herself in the past, though she was a teenager and we are now a few years out of college and now she only suffers from more mild depression).

So…what role do I play here? She obviously needs me since I am her best friend. Do I encourage her to tell her boyfriend the truth? Tell her to dump her boyfriend? She claims that she doesn’t want to because she does love him and that she now knows for sure that she doesn’t want anyone else (she does not like the guy she cheated with).

Do I tell her that what she did was okay? Tell her that we’re all human? Tell her to never tell him (No, I do not plan on telling him)? I don’t know if I could do that, given my feelings about cheating (that it is always wrong, and almost unforgivably humiliating to the other party).

Maybe it’s none of my business at all, but I just can’t stand seeing my friend so upset and feeling so much self-hatred and am worried about how she will react given her depression. I am as shocked as anyone else that she did this and while it was wrong, I feel the need to support her in some way. What does the best friend do?