Your concern must be solely for your daughter, the guy shouldn't enter into your worries.
Arrange to send your daughter, and some friends of the same sex if possible and absolutely without her intended, somewhere for a week, two weeks, a few days if nothing else. Disneyland, Nantucket, San Francisco, Detroit if nowhere else. Somewhere, anywhere. Get her out of the house, solo, for a time. If possible hide her cell phone during her outing.
Both she and her intended can re-screw their heads on during this time. Whatever it costs, it'll be worth it one way or the other. Think of it as an investment in her future. Both she and the guy have to work out their feelings by themselves, you can't do it for her as much as you'd like to do so.