Would i be in the wrong to not show my support for my best friends engagement?

I introduced my best friend, which is a female, to my other friend. The first night they got intimate with each other and after that they became an item. Two weeks later he proposes to her and now they are acting inseparable. My first issue is the fact that they have not known eachother long enough to be getting engaged. It may not be my place to make that decision but in all reality i know my her very well, and there is so many things that he does not know about her which is why i dont agree with the marriage. i feel like i would be considered a bad friend if i chose to not show support which is what is killing me. not only does he not know her like a husband and wife should, but i know for fact that she has cheated on him multiple times after the engagement! i caught her in the act with her ex which is also her baby daddy.the thing that bothers me is that ive seen my parents marriage and how hard it has been for them. many issues including infidelity have came up, so i dont think that anyone should go into a marriage if they are truly not ready and i dont believe she is ready. i know that is not my call but how can she have the audacity to cheat on him three times with two different guys after he proposes, and then still get married under the law and under god. it just doesnt seem right to me and i want to voice my opinion, but i dont want to seem in the wrong. what should i do? and just some additional info: she has never been faithful to any man in her whole life. NOT A SINGLE ONE! she has a reputation in our area as being a hoe. I even tried to give him a subtle warning about her without putting all of her business out there but he did not listen. i honestly feel bad for the guy because he just got deployed today, and he has left thinking that he is her only one when she just had an affair a week ago. and i dont want either of them getting into a marriage thats based on lies. i could ramble on and on about how much this isnt the right thing for them to do. what should i do? support my best friend even though i know her better than anybody else and know that she will not be faithful to this guy while he is gone for 7 months? or should i voice my opinion and tell her that this is wrong?