I can understand a certain group of brides do this - usually those who cannot afford or don't want to afford a proper wedding pro and went with the bridal studio assigned budget ones.
They never know who will turn up on their wedding day to shoot coz the bridal studios got a pool of low end shooters the bridal shops really can't commit either.
Thus these couples feel a need to get a friend or relative to shoot. Actually, this practice was actually passed around on bridal forum groups. The practice is stemmed from fear arising from all those horror stories about bridal studio AD photogrs.
But the real wedding specialists often have requirement for the couples not to ask friends or relatives to shoot - for good reasons discussed in the past 7 pages of this thread. Might be good to read before posting another similar response already discussed earlier in the thread.
You never know if your friend/relative hired a real pro, a bridal studio budget shooter, or any Tom dick and Harry off Clubsnap hobbyists.
So best not to shoot - you might be messing up the real work the couple hired for.
If the wedding couple asked you to shoot, think for a moment, are you Tom, Dick or Harry?