A few weeks ago I had a sigmoid resection (LAR) and was first given water, then broth, then a ‘low residual’ diet. Your mother might be put on a BRAT diet ( Bread, Rice Apples, Toast https://en.m.wikipedia.org/wiki/BRAT_diet
) before being given the go ahead for the low residual diet, especially if she’s had issues with constipation in the past. The idea is to give the colon time to heal before making it work, and to make its job as easy as possible by only eating easily digestible food until well-healed.
I’ve only just been given the go ahead to introduce veggies and fruit to my diet, and I still have to avoid fibrous meats. My surgery was laparoscopic, so I’m not sure what additional restrictions an open surgery would have if that’s what your mom had.
Hoping for the best for your mom!