It shouldn't, but it depends on how sensitive they have it set. The metal detector works by a change in magnetic permeability on the inside of it (the part you walk through). The detector is (usually) a big coil that you step through.

Platinum is about 13x more magnetic than aluminum, for instance, but only about 3% as magnetic as iron oxide. These detectors are really designed to detect steel.

I have a platinum bracelet, that probably has more platinum than your fiancee's ring (because of its size) and it has never set off a detector -- and I fly a LOT.

Good luck!

Jim