I have been waiting for a thread such as this! DD 9 has been battling cancer for the last 4 years and I have always wondered if I should drastically overhaul her diet.
I second Angryfeet's suggestion re Anti-Cancer book by Servan-Schrieber. I love it, it has so many suggestions and details about what sorts of foods are good, such as multi-coloured fruit and veg, crustaceous veg, green tea, garlic, and so on. And should avoid dry cleaning as too many chemicals used, pesticides etc. And it struck a chord with me as he wrote about his own experiences battling brain cancer. I highly recommend it to everyone!
I bought another book called Beating Cancer With Nutrition but haven't finished reading it yet, the early pages seem a bit rubbishy to me.
Sugar is supposedly a big no no as cancer feeds on it.
I have read somewhere that milk and cheese should be cut out...also acidic foods because they provide a good environment for cancer to grow. alkaline foods are preferable.
Soy is to be avoided unless it is in a fermented form such as tempe.
Cured or processed meats are definitely a no go...such as sausages etc. Best to make your own burger patties.
Supplements/juices made of green veg such as spirulina or wheatgrass are supposed to be good...
Does anyone know if going organic makes any difference? I try to buy organic stuff nowadays but can't always find it, or afford it all the time.
Anyway I have no idea if any of this actually works! DD is very erm independent-minded and is vehemently opposed to eating vegetables and will only eat certain types of fruit. And she loves her chocolate. I mostly let her eat whatever she feels like eating but I try to make her eat a variety of different things. Although it can be hard to implement this rule.
She also used to drink lots of milk as a toddler (no-brainer there I guess!) and loves cheese and edamame soybeans...the only veg she will happily eat without me forcing or nagging. I don't know if that is a coincidence or if it proves the theories 
Troubled, if your mil is not eating much, maybe you can try nourishing soups to keep up her energy levels?