SoCal here. The horses that were locked in their stalls during the fire in the Valley a year back - died in their stalls. The horses at the TB layup barn near Del Mar last year - ran back into their barn and some died. Gate locks, stall locks are false security. Practiced thieves carry bolt cutters. Horses also die from smoke inhalation - in their stall or at large.
Sometimes you have time to evac in a fire. Sometimes you don’t. Sometimes you have to run so fast you leave with just the clothes on your back. We’ve evac’d horses, dogs, cats, chickens, goats - three times in 50 years. It is never fun. The worst is when you’re trying to load a horse with smoke, sirens and flashing lights around you. Vick’s in their nostrils helps.
What you do want to know is what the BOs plan is for evac. Trying to match up keys to different locks in an emergency sounds impossible to me. Seconds count. She should likely consider buying locks that she has a master to and just charging the owner for the lock or ban locks. You also want to look at the area around the facility to ensure there is good fire clearance and plenty of hoses and fire extinguishers accessible for all to use.
I would ask other owners why they’re locking their horse in its stall. It may be a case of so & so did it so I thought I would too.