I have been a farmer and gardener most of my 60 years and we kill plants and weeds with salt. Epsom salt is a salt. Your mother may have a special formula or process only she knows about and I am not familiar with what she has in mind. I would carefully inquire how she would use the Epsom salt.
I've never heard about putting epsom salt on plants to assist in growth and yield however If you put an aspirin in the soil its supposed to help it grow. Or the topsy-turvy plant holders work!