How do you tell someone their partner is cheating?

I was at a bar last weekend with a few mates of mine and I saw one of my best mates girlfriend, G, dancing. G was dancing with some bloke who I thought was my friend, so I edged through the crowd to go talk to them. As I neared closer I noticed the man in question wasn't my friend at all and G was kissing the guy. I was pretty outraged and was honestly going to go over to him and punch him in the gob, but I figured he didn't know she was in a relationship, afterall it's happened to me a couple of times.

I sort kept it quiet over the night, trying to think it over a bit. I've only told one other person, my friend Hannah, who is a friend of G, and she told me she has heard of other random hook-ups over the last couple of months. I was pretty surprised by this, G seemed like the honest type.

So in essence I figure I have two courses of action:
1. Tell my friend directly what I saw and what I've heard from others.
2. Talk to G first and maybe get her to own upto it.

I was more inclined to go talk to G and get her to discuss it with my friend, but this action has the possibility of her toning down what she's done and how often it has occured. So I guess I'm asking you, GFF, on the appropriate course of action. If you have any other suggestions to consider, I'll take them onboard too.

It's pretty heartbreaking stuff, he was talking of plans of marriage in the near future.

Jam it back in, in the dark.

I don't know anyone else who's seen anything that would care about the situation. I was thinking about him not believing it, but I'd say he knows I wouldn't lie about something like this.

There's nowhere I can't reach.

I honestly don't think I can keep quiet for too long. Also she already hates me so that's not a problem.

This thing is sticky, and I don't like it. I don't appreciate it.

The problem sorted sorted itself out, he caught her in the act.

Last night we went out, he wasn't going to come because he had work in the morning, but he agreed to pick us up. When the time came that we wanted to go home we were in the club and we asked to come in for about 5 minutes while we say goodbye and finish our drinks. When he was waiting I saw her dancing with a girlfriend, I told him she was over there. "She said she wasn't coming out tonight. I know what'll be funny." He then pulled out his phone and called her while he was watching her, he was slowly walking over as to surprise her when she was talking, but she looked at her phone and cancelled the call. Then some other bloke came over to the 2 girls with drinks in hand, then she started kissing the bloke.

My friend just ran up there and punched him right in the face, he fell back knocked over a table full of drinks and fell to the ground. He didn't get up, he was knocked out. She tried to apologise and hug upto the him, but he just walked away. Loads of people in the bar were cheering.

Needless to say we had to find another way home, but I spoke to him today and he seemed pretty okay. He said he kinda had a hunch something was going on, but didn't want to believe it. He was pretty satisfied after I told him the guy was out for about 10 minutes.

But yeah, I guess I got off the hook a bit. I had made my mind up I was going to tell him, I just didn't know how I was going to bring it up.

Additional Spam:

Originally Posted by Decoy Goat

She has a pretty unique name so chances are when he's googling her name he'll find this thread and you won't have to go to the effort of telling anybody~

lol good point, I'll change her name in the OP.

I am a dolphin, do you want me on your body?