Is it permissible for a sick woman to use money collected for her for a surgical operation, to spend it on post-operative treatment supplies, food, and drink, knowing that the amount has reached the nisab and a hawl has passed over it?

If the donors stipulated that the money be spent on the surgery, then whatever is not spent must be returned to them. However, if they did not stipulate anything, then the money is considered the property of the woman, and she may dispose of it as she wishes for her treatment and other needs. She must pay zakat (definition) on it if it reaches the nisab (definition) and a year has passed over it.