What if you saw your friend's BF/GF cheating on them?

Finally I've got the time to write bout something other than my social life.

Ever been in the shoes where you're stuck in between? An example would be prostitution. If you say prostitution is dirty and low life, people would say that you're discriminating, But if you say prostitution is legal and acceptable, also an honorable job, people would look at you differently, in a bad way of course. Either way, you don't win.

So here's today's topic : What would you do if you saw your friend's BF/GF cheating with another girl/guy?

• Confront the BF/GF on the spot and ask what are they doing and who is that person they're with. It sounds like you're being over protective of your friend. Who are you to butt into his/her business? It's really none of your business, no matter how tight you guys are. Btw, by doing so, you're indicating that your friend is a weakling and have no abilities of defending him/herself, plus not respecting them at all.
• Stalk them and report to your friend once you get home. I most certainly will be choosing this option, it's just fun to stalk, and definitely fun to tell. But the worst part is, what happens if the cheater saw you and reported to your friend in advance, proclaiming that you were trying to sabotage their relationship. Anyone blinded by love,would definitely not listen to what you've got to say. You might as well be sabotaging your own friendship. So is that really a wise decision?
• Tell/discuss with everyone except your friend who is being cheated on. This sounds so so so not good. What happens when your friend finally found out from another party, that you've been spreading stuff bout his/her relationship? He/she will be so devastated that you wouldn't be considered as a friend anymore. Betrayed by their partner, and their friend. Enough is said.
• Keep it totally to yourself, your friend will know the truth eventually. That's the safest way you can think of. But that's not really being a good friend are you? Knowingly allow your friend to be played like a fool when you could've said something, classic.

It's hard enough being in that situation, but what if, you're best friends with both of them? You're friends with the one cheating, and the one being cheated on. Which side do you stand on? Can you even choose?

• Side on the same sex.
• Side on the one who's the victim. (But which? There's always 2 side of the story.)
• Side on both, and be a 2 faced fucktard.
• Side on neither, it's none of your business anyway.

The last option is fine enough for me, but what if, both side confides in you. OMFG!!! That's like putting yourself in a position where they want you to split into two pieces!!! That's why I think that sometimes, friends don't really care bout you, they only want someone to hear them, and solve their problems. So? Does this means that friends are meant to use each other in that way??? That's only what I think, though.

Last question, would you want your friend to tell you that your partner is cheating on you? Or you're better off living in a perfectly happy lie?