Hello and welcome to healthcare forum.
As the problem of anal fissure
is long standing and not completely relieved by medications, the advice of operation by the doctor is appropriate and the operation done for anal fissure is safe and not a major one.
Regarding diet she requires high fibre diet with plenty of fluids orally and its better to avoid junk and spicy food.
For pain relief local anaesthetic
ointment will work.
Best wishes for health of your mother.