My brother is 6'5" and slept fine on the beds in the Sensation. We had an upper in our cabin and he got up on it visiting us and was fine. He had a regular bed in his cabin.
When you are tall you just get used to your feet hanging over the end of the bed and it doesn't bother you to much.
Depending on what ship you are sailing upon, if he uses the sofa bed, you could put a pillow on the small night stand and he might have ample room with his head against the wall. The sofa bed is at least 6 ft with no arms.
We were on the Triumph, and my son is 6'1". He slept on the top bunk, and his feet hung off the bottom if he laid straight out. If he curled his legs up a bit he was fine. Forget about the rollaway or the sofa bed. My daughters (around 5'4") slept on both, and their feet were hanging over into my bed, I had feet in my face every morning. !!!
They are both rather small for sleeping.