Short answer: apart from the usual healthy diet recommendations, whole grains and coffee!
A recent systematic review and meta-analysis by Chan et al. (May 2024) found that: "[...] The overall evidence was graded as 'limited'. The inverse associations between healthy dietary and/or lifestyle patterns (including diets that comprised plant-based foods), whole grains, total, caffeinated, or decaffeinated coffee and all-cause mortality and the positive associations between unhealthy dietary patterns, sugary drinks and all-cause mortality provided 'limited-suggestive' evidence. [...]."
Update 3 May 2025: A new meta-analysis by Chen et al. comes to a very similar conclusion: Among people with colorectal cancer, all-cause mortality (i.e., the risk of an earlier death) appeared to be decreased by when the intake of whole grains and coffee was higher, alcohol intake was moderate or low [for example, about <450 ml of wine], and especially physical activity was higher.
Conversely, a high intake of refined grains and high-fat dairy products, as well as smoking and television watching appeared to increase all-cause mortality risk.
Calcium supplementation appeared to lower the risk specifically of colorectal cancer-related death.
The authors concluded: "To optimize [colorectal cancer] survival, [.] patients may be recommended to adopt a diet rich in whole grains, coffee, milk, and dietary calcium; but to avoid excessive alcohol, refined grains, high-fat dairy, sugar sweetened beverage, smoking, and sedentary lifestyle; and to engage in regular physical activity while maintaining a healthy weight."
The recommendation to drink more milk but to consume less high-fat dairy might seem confusing, but may just suggest that the dairy fat is "bad" and the calcium in dairy (or non-dairy vegan sources) is "good" - at least in the case of colorectal cancer.
In summary: The usual healthy lifestyle recommendations are also a good idea for people with colorectal cancer. Eating more whole grains, instead of refined grains, drinking coffee (for example, one or two cups daily), and taking a calcium supplement (for example, about 300-500 mg/day) seem to be good advice - based on current evidence.