@sozobe,
Also...
We don't know where they're going with this, where they will wind up.
If they do this half-hearted, yeah-maybe, I-don't-actually-like-you-that-much-right-now wedding, with the party and the gifts and everything, and then once the stress is off find their groove again, and then are totally in love again, and then REALLY want to get married... then what? Will they have yet another ceremony where they really mean it? Or will they just have the wedding pictures where they can't muster a real smile, and the vows they uttered through clenched teeth?
If things are really not looking good (what's the latest, helpasap?), I think I'd recommend something like sending out a notification to everyone and saying that while there won't be a wedding, there will be a party that they are more than welcome to attend, (no gifts please). That way the ones who already have plane tickets and hotel reservations and plans to see family/ friends can attend
something, while the people who didn't have to travel far can just stay home, if they want (they could still attend, too). And everyone can just return their gifts (or not buy them if they haven't gotten around to it).