You did not say if your mum is having chemo or had surgery so I am assuming she still has cancer. What you/she needs to watch out for is not to eat foods that can cause inflammation as it will cause problems with pain and lead to more problems as the cancer swells. Other foods that can quickly covert to sugar (simple sugars) will feed the cancer and can help it grow. Carbs such as white bread are one of those items that can quickly be converted to sugar but if she is on chemo she may need to eat this type of carb.
I have a blended drink of oranges, peaches, bananas, assortment of berries, soya milk (no-sweetened) and whey powder. The whey power has protein and other nutrients you body needs. I found that it helped me stop losing weight. There are a number of books out there that cover what to eat if you have cancer and what to watch out for (as noted above). There is also the internet that has helpful advise. The most important point is that she may need to change her diet due to the cancer. I have read many of these forums where people are suffering due to what they are eating.