It depends on the cat. Feliway is a big help and can take a couple weeks to see a difference. I, however, used Zylkene (supplement and natural, no meds in it) to help Lexy as she was afraid of her own shadow at one point, and Feliway had stopped offering help, she was always under the bed and jumpy. Zylkene after a couple weeks has turned her into a little kitten at 7 years old. She is still skittish but less so and rarely hides anymore.
Hope it all goes well at home, it must be distressing to see.