My ex wife's fiance wants my son to be the best man at their wedding should i allow it?

My ex wife is getting married to her fiance in August we have two kids together 2 boys ages 7 and 13. When my ex came over my house to pick up the kids she asked me if my oldest son Max could be her fiance's best man at their wedding her finance seems like a nice guy but i told her i will think about it but i was wondering why didn't he just asked me face to face like a man?