When you look at a mainstream study you always have to consider who funded it, what type of vitamin/mineral/supplement did they use and how much or how little did they use.
Selenium is not like smoking cigarettes - it is an essential element our bodies need. It also comes in more than one form and the most common one is produced as a waste product from mining operations for metals.
The items you listed are all good microbe killers, and I think microbes are a key - but not in the sense that all cancers are caused by viruses, or bacteria either. In my opinion, cancers are likely caused by irritation from toxins or even microbes that enable microbes to get inside the cell, which results in a reaction that changes normal cells into cancerous cells that gain a foothold and continue to multiply without incurring normal cell death. Cancer microbes are, in my opinion, pleomorphic (sorry Pasteur, but you blew it!), and can change from virus to bacteria and back again.
The very best microbe killer is colloidal silver, especially if you take good enough quantity and quality. Two other good ones are grapeseed extract and oil of wild oregano.
Ultimately it will be her own immune system that will beat cancer once and for all and that will require establishing a good foundation of a healthy diet and lifestyle choices. Supplementation can help fight against cancer and build and boost the immune system at the beginning, but in the end it will take a commitment to maintaining a healthy diet and lifestyle,
Here are some of the best immune boosters besides oleander (which is the best one of all).