I am very sorry for you and your family to hear that your mum is so ill.
Some of the prognosis depends on whether the cancer of the stomach is a "new" growth or is related to the breast cancer. It would be an unusual place for metastasis but stomach cancer could have been a way of putting it to your mum, saying that is the place it is, but actually it could be a metastasis from the breast cancer near the (organ) stomach.
Quite often chemotherapy can help a lot, even if it does have sideeffects. Then if the growth is reduced in size it maybe can be operated successfully. All in all it is a serious illness though.