,there is chance of lymphoedema of corresponding upper limb with swelling and pitting.
IF it gets infected,it will become red or purple with local rise of temperature.
Please give her good antibiotics,if possible two,anti inflammatory medicines .
She should keep her limb elevated.
It will come down.
But,the edema if at all due to breast cancer
will be there .You should see that no injury
occurs in that as it may lead to infection.
Wish her speedy recovery