It depends on them really and their sense of humour. I'd be tempted to put something to show how pleased I am that they are back together ( if you are). My sister married her third (!) husband who was an ex bf from 30 years before and a friend made me a 'two peas in a pod' wedding card, I wasn't crafting then. Inside I just put Perfick! It's now framed and still on the wall.