Your mom could potentially sue either the guard or the employer.1) When person either deliberately or carelessly (negligently) injures another person, the one causing the injury may be at fault or an liable for the harm he causes. Pushing your mom out of the way could be an intentional or negligent act (depending on circumstances) giving rise to liability.2) An employer is liable for the actions of its staff done in the course of employment--so, for example, a security guard or other security staff member chasing someone as part of his security job.However, what you can sue for and recover is limited to the harm suffered--so, for example, medical costs and a lost day's wages, which may not be worth actually suing for. It might be worth contacting the employer and seeing if they'll agree to give you mom something as settlement for her trouble and pain, to avoid and possibility of being sued.