Most places of employment that I've seen with lockers generally do not have issue with the staff using them and securing the contents with locks.

Word to the wise, better to ask up front before making presumptions regarding potential surprise policies on locker use.

Some places only have enough lockers for one shift and the shifts share lockers, which means you bring your own lock for use during your shift and leave an empty, unlocked locker when leaving for the day. Others may have restrictions on what may be kept in them, and for how long it may be kept. Other places may require that you leave a key or combination >To your lock!< with them for use "in case of emergency". Hopefully cooler heads and policies will prevail. YMMV

Glad to hear that she at least has access to a locker, hope it is relatively close by her work area and that it remains accessible in a time of need.