Hi - first of all may I invite you to log on to:-
http://www.medhelp.org/posts/show/523166
where I describe my own experiences after a total colectomy.
In this posting I also include my own approach to eating after the operation. I followed this diet at the time mainly to reduce the "liquidity" of my motions and avoid anal soreness arising from the corrosive nature of the liquid motions.
Lastly it may be worth your mother asking her surgeon if she could be suffering from a lingering ileus (see my posting for a further explanation) which might cause a reduced appetite.
Do come back with any further questions you may have
Good luck
Morecambe