I agree that the details of the situation are highly gendered, because frankly, male friends do not go on diets together with any near the amount of frequency that women do. Nor do they have such a complex social language of dress to negotiate afterwards.
However, if the bridegroom and his friend had gone on diet together, and his mate had a decision to make between smart wedding guest attire and a suit that showcased his weight loss to maximum advantage, he would naturally choose the former, wouldn't he? Granted, I doubt if there even is such a thing for men, but there is for women, and therefore social rules apply to it.
What I'm trying to say, is that I don't agree that 'don't be too jubilant about your success in your mate's face at their own party' is a special burden that women have to bear. I've seen 18 year old boys balance being thrilled with their own A-level result while supporting less successful friends at A-Level results day, for a start.
To be honest, as I've been dress-shopping this week, I am quite inclined towards the idea that the variety in women's attire and graduation of dress-codes thereof (cocktail party, formal, wedding, black tie) is oppressive, and an unfair burden on women, etc. My opinion is not the only valid opinion on this matter- women who like dress shopping may feel that women have a vastly better deal than men, and be happy that we have greater provision
Regardless of anyone's feelings, clothing is a language, and women speak Japanese at events while the men get Esperanto. That is a tad unfair.
However, this is supposedly the OP's friend. If she was mine, whatever my personal resentment of Japanese grammar, and pronunciation, I wouldn't want to risk committing the sartorial equivalent of a glaring grammatical error and formal discourtesy to her on her wedding day.