State laws say that disabled persons who have a disability parking placard are the only people that can legally use them.

Note, though, that they can use them and receive parking privileges as either: a driver, or a passenger.

This means that a disabled driver can use a placard in another car, but he/she has to be present in the car when the tag is displayed.

For example, a disabled driver can use a parking placard in another family member’s vehicle. He/she can use it as either the driver, or can sit in the car as a passenger and let the relative drive. If the latter, and the relative is the person parking, then he/she is authorized to park in a handicapped parking space.

However, a disabled driver cannot “loan” a disabled sticker to a family member so that he/she can secure a close parking spot at a parking lot for the person’s own convenience.

The misuse of a disabled parking placard is treated slightly differently among all states. However, it is generally considered an infraction and can get punished by means of:

a fine, the revocation of the placard, and/or community service.

https://www.shouselaw.com/ca/blog/can-you-use-your-handicap-placard-in-another-car/

Seems like it's a no. Probably not worth it to lie over this as it means your mom might lose her placard. Take the fine and move on.