If you are worried about her getting loose, or if she is worried about it, then have her do Kegel exercises regularly. Maintaining pelvic muscle tone and strength with ensure her pussy remains tight - perhaps even tighter than before - yet still able to stretch to accommodate your fist.
A sensible suggestion , she will enjoy that "fullness" and have tremendous orgasms, and done regularly shell become deliciously loose/looser, shell need reassurance being loose isnt a bad thing though