Since it's new, it should be under warranty. Contact the merchant you bought it from, they may be the ones who have to deal with Giant. Giant does have it's contact information online, but I don't know if they're responsive to consumers. This is the listed contact info: Giant Bicycle, Inc.
3587 Old Conejo Rd.
Newbury Park, CA 91320
I installed a Currie conversion kit this summer. It has a rack mounted battery case that locks, too. The locking bar is fairly good-sized, but the battery box is plastic so I suspect a determined thief could get it out. The Currie battery isn't LIFEPO4, but just SLAs so it wouldn't be as attractive a target.
Good luck. That bike wasn't cheap and Giant should make things right. (How big was this bike enthusiast? I'm glad my battery case is obscured from view by my panniers.)