Most of those studies are either BS, guilty of pulling a Chicken Little or the result of an uninformed journalist who misinterprets what the actual study is saying.
The say stuff like eating XYZ increases your risk of getting ABC-type cancer by 20% when in fact the chance of getting cancer for those who don't eat XYZ is 2%. If you eat an above average amount of XYZ, your cancer risk is now 2.4% (20% increase) instead of 2%. Scientists have a term for this. It's called statistical insignificance.
The largest factor to you getting most forms of cancer are errors in DNA replication. The darker hue below for Environmental are HN (head and neck), E (esophageal) and L (lung) and due to smoking. The M (melanoma) is due to sun overexposure. The U (prostate) needs more data.
The major cause for colorectal cancer, that you described about your friend, is primarily in the Replicative column. Although there's a slight increase on the Environmental side for stomach cancer, North America and Africa (diets that you'd consider extremely rich) have the lowest incidents worldwide.
DNA typos to blame for most cancer mutations
How Much Do You Really Have to Worry About Cancer?
DNA Replication Errors Contribute to Cancer Risk