A friend has been told they have colon cancer, diagnosed via a tumour marker blood test, high cea, CT scan showed contained mass. Is this conclusive?

No, but... But very suspicious for colon cancer. If your friend is similar in age to yourself a mass in the colon on ct scan is cancer until proven otherwise. And cea levels can go up for other reasons but, given the mass, they would strongly favor cancer. But to answer your question "no"... They are not definitive. You would need a piece of the mass under a microscope to prove definitively its cancer.
Biopsy. Biopsy of the mass at colonoscopy is the only conclusive test, but the list of abnormalities identified is strongly concerning for colon cancer. Best wishes.