I wonder if they've decided that neither of them will never go anywhere socially without the other? It's not a choice that I would make, but some couples do.
If that is in fact what they've decided, they are going about it wrong. When only half the couple is invited, they need to decline the invitation, not bring the other partner anyway.
If Sarah were to ask me why she is no longer included, I would be honest. It's possible that she and Bob have already alienated other social groups since Sarah is so intent on being included in this one.