My fiance have a transverse fracture in his foot what would happen if he doesn't get it fixed and kettle it heal on its own?

Depends: A non displaced transverse fracture will heal, but will need immobilization of some sort such as a cast or cam walker. If it is displaced then in needs to be surgically fixated so that there is bone healing. If left alone, this may lead to a non-union and nagging pain.

Depends: Not all fractures require surgery. If a fracture is in good alignment, the best course may be to immobilize the foot in a cast or boot. If his fracture is in poor alignment, if there's gapping, or if a joint is involved, surgery may be required to aid in healing and prevent painful arthritis in the future.