Pressure around the orbits (eyes) and backhead due to the strap of the dive mask could be contributing to the swelling and the antihistamines will prevent the swelling from getting worse. It will take its own time to go down and if it is not worsening, it is certainly a good sign.
The other possibility (although remote) is an allergic reaction
to the material in the mask and commonly latex allergy
. However, this would have involved a more intense reaction with itchy skin around the eyes, sneezing
and (if severe latex allergy) runny nose with breathing difficulty. If something like this happens in the future, do ensure she gets tested for latex allergy.