It's very difficult for any of us to give you advice or say what will happen. If she is undergoing cancer treatment, then there is definitely a chance she will be deemed medically inadmissible. Ideally she should complete all of her treatments and be declared cancer free. However this would require you to withdraw / cancel the application and apply for the lottery again later once she is healthy. You could also take the chance and have her complete the medical now. There's no easy path here unfortunately.
Your mother’s oncologist needs provide information on her disease type and stage, current treatment, treatment plan (medication and how long), whether surgery is necessary and what type of surgery although that is really the domain of a surgical oncologist, how many scans are likely to be needed and what type of scans and lastly what is her prognosis is. Unfortunately with cancer treatments there tend to be bumps in the road during your treatment like having to treat low blood levels (meds can be expensive), you may need an unplanned hospitalization, etc. which makes calculating cost of treatment a bit tricky. Your mother will have completed part of her treatment in her home country so that would also be a factor. Is your mother over 65? When it comes to approval it will depend on many things like the type of cancer and stage, how much of the treatment has been done in her home country, what type of surgery is needed, the cost of the chemo, whether radiation is necessary, etc. It is possible she could still get approval especially if the cancer at an ally stage. I would speak to your mother’s doctors and ask them to start preparing the information I listed so that she can submit more information on her health during her medical. Unfortunately most doctors charge a fee for letters like this.
