The chances of the lump being a malignant tumour are actually quite low - it's more likely to be a benign tumour or something like a cyst. Try not to worry too much, and be supportive - make your mum a cup of tea when she gets home, because she'll be more worried than she needs to be.
As far as I know, if the lump does turn out to be cancerous, there are a few options open. She'll probably have the lump removed and then have radiotherapy or chemotherapy to get rid of any remaining cancer cells. But, as I said, try not to worry too much as it may not come to that.
Chin up, and be nice to your mum.