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.
Epsom salt works especially well with tomato, rose, and pepper plants. Add a tablespoon of salts to one gallon water. Water plants once a week with this for best results. Use this as a spray on plants when they begin the flowering process and when fruit forms.
Absolutely it will help to keep bugs off from them as well as keeping fungus away from the plants it also help to put nourishment into the soil so that the plants will grow wel. And it will make sure no slugs get on them.
Epson salt is excellent for tomatoes, peppers, lawns, shrubs, and roses. Epson salt is not a salt. It is magnesium sulfate. I used 1 tablespoon per gallon and spray it onto the plant over two weeks. Its a cheap fertilizer.