I'm unsure why I can't respond to your comment under my post but apparently I can respond here. Apologies in advance, I'm on mobile so the formatting isn't going to be the greatest.
First and foremost, I did not say anything about OOP's judgment of character. I fully believe OOP is a victim here.
Your statement of, "trust someone you don't know very well" is highly reductive. It may be true that OOP's fiance didn't know OOP's best friend very well, but that's a symptom of a deeper issue. There isn't much mention of how well they know each other, therefore, almost all statements of this nature are essentially conjecture. However, they do go on double dates together, so they definitely know each other to a certain degree. Again, I make no statements based on my perception of her judgment of character.
It's very weird the first thing you do was make up things I did not say under the guise of, "making things clear".
Also, OOP did not know her fiance all her life, she knew Matt since childhood, the one who is together with the best friend. I'm not sure where you're getting that OOP knew her fiance her whole life, but that's false. Then you use this line of reasoning, which is already false to say, "trust someone you've known your whole life over someone you barely know.". If I didn't make it clear earlier, I'll do it again here, this is such an outrageous statement, I don't even know where you and others have come up with it.
Trust someone you've known your whole life
OOP didn't know fiance her whole life. She knew Matt since childhood, the guy she was supposedly pining after for her whole life.
Someone you barely know.
Not sure where you read they barely knew each other. Most people would assume two people in a relationship would know each other's best friends to a degree above "barely know". Even more so if you go on double dates with said people. Secondly, due to the position of trust they would hold in your partner's life, it's completely disingenuous to disregard her at the same level as a stranger, because, well, they're not.
Trusting a near stranger.
How detached are you from society where you think it's normal for your fiance/e's best friend to be a near stranger. I don't see any mention from OOP making such statements, but it seems you've taken the liberty to make that canon.
If someone told me...
Ah yes, how easy would life be if we could really prove the existence of things by their absence. How does one prove that they're not really marrying you solely for your money? How does one prove that they're not using you as a placeholder for another guy? Normally, people don't gather evidence in advance for a situation they're not anticipating. Any and all proof would be purely their word against someone else's. What if the nature of the statement calls into question the loyalty/honesty of your partner?
What if your fiance really was cheating on you, only with you for the money, etc.? What if the best friend really was telling the truth but they didn't have any proof besides hear say? However, here's the kicker, Matt, was also saying OOP has been pining after him since forever. So now you have two people who are saying the same thing, that OOP's fiance is just a placeholder, who do you believe?
Imo, another weird thing you just happened to overlook again, but not particularly outrageous. It's extremely native you'd still believe your partner over all that, especially when multiple people are saying the same thing.
You could be right, it could be confirmation bias. I thought I worded my response in a way such that you couldn't disagree to any one part without agreeing to the other part first.
It's not that I don't believe anyone says or does proves my point, but I believe empty disagreements like yours do as the entire post was actually about armchair Reddit psychologists who can't see the irony of calling out the best friend for toxicity while being toxic to the fiance at the same time.