My friend was recently diagnosed with stage 4 colon cancer. Is cancer treatment always trial-and-error, or are there tests that help doctors choose treatments more precisely?

Treatment for colon cancer is guided by internal guidelines like NCCN and ESMO guidelines. The guidelines are based on robust clinical stuidies. For stage IV disease, the molecular profile or characteristics is being analyzed to determine what set of drug will most likely work. Just like in any test, no test will give a 100% assurance that a drug will work. The condition inside the body could not be replicated 100% in the laboratory. These conditions that are unique in every individual will affect the final response to the drug. That is why given the same cancer type and same test result, a drug  will work for one patient and will not work on another patient. The treatment recommendation is based on robust clinical evidence and guided by molecular profile of the tumor. But then again, no two individual and even no two tumors are exactly the same explaining different responses to different patients.