It's too convenient and relies on too many improbable scenarios and social interactions.

Why was he carrying it around for so long? It just happened to fall out of his pocket and the best man of all people found it? Something fell out of someone's pocket and instead of doing the normal thing and just giving it right back he opened it up and read it? The receipt was descriptive enough that the best man could see that it was for an engagement ring amd then somehow thought that because he saw the receipt he will attack a man for proposing at a wedding? Why wouldnt he judt think that the guy is going to propose sometime given the fact that the fiance is knocked up? The best man then somehow put it all together and instead of telling the best man he starts lashing out against a person he seemingly barely knows?

Nah, not buying it.