Sometimes when I tell wedding clients that they need to ask their guests to not use flash photography at a wedding I get inquisitive looks back in response. When I ask the DJ to announce it at a wedding many guests don't like the fact that they can't use flash. The unfortunately assume that we're asking just because we don't want them to get shots. That isn't it at all. I can't tell you how many shots are taken where shutters and flashes fire at the same time. So when guests fire their flash it can ruin a shot and possibly spoil a rare moment.
I highly recommend that all photographers prohibit the use of flash photography anywhere they are shooting. I don't recommend being contentious or nasty about it, but merely inform your wedding clients that things like this as shown in this shot truly do happen frequently. I don't ever require it, I merely inform. 99% of the time when I show my wedding clients shots like this one they understand and they do all the enforcing at the wedding. Hope this tip helps!