Is my girlfriend hypocritical

So hypocritical ... should I say something?

Hello, I don't really know whether to go here or there and here I can at least write in black because I am uncomfortable talking about it.
So, it's about my best friend. She and her fiancé are getting married next weekend. Since she was pregnant (the little one is now one year) (they had been together for 8 years before) he started cheating. At first it was just text messages, then letters, then he stayed away after work, supposedly to drive colleagues home and so on. It was very obvious what was going on, but because she was heavily pregnant no one wanted to say anything. After the birth, everyday life seemed to return and she told me that she suspected it, but no evidence. Although I think it's a text message like "I'll be back later, she has suspected, I'm looking forward to you" says something, doesn't it? No matter. In case of doubt for the accused and on Christmas Eve she told me that they wanted to get married.
Peace, joy and pancakes.
The whole thing started all over again in January. To & quot; the whole & quot; to judge a little more precisely, I sent my friend off with him and what happened? My friend's fiancé attacked one woman after the other and asked for phone numbers & quot; for later & quot; like he said. Not awesome I almost fell off the sofa when my boyfriend told me.
Now I tried as best I could to stay out until my friend called me last week, in tears and said the little one was sick, her fiancé was not at home, not at work, cell phone off, whether I could come and something from the Could pick up pharmacy for them. Clear. I did it and then waited with her until her fiancé came home (at half past one in the morning). He would have had a drink with friends, battery empty.
Another big question mark.
A few days later (on Saturday) & quot; the same & quot ;. My friend called and asked me specifically if I could keep an eye open while I was out with friends anyway. I did and saw something too. Him with someone else, holding hands and kissing.
I asked him the next day when I was visiting my girlfriend and he said I must have forgotten, he had been with his brother all evening.
My girlfriend seemed to have forgotten the matter, but I didn't and now I don't know, shall I say what and end up being stupid? Or do I say nothing and let them run into their disaster? I think she would marry him anyway, even if she did, but do I want to be the one to get the message across?
We're not talking about & quot; children & quot; here either. He is 32 and she will be 30, so already grown up and you should think a little smarter.
You can't help me either, but it's good to have the thing off your chest.