If you know information that could potentially help someone, why would you withhold that information? Should a dietitian be the only person allowed to tell someone that consumption of fast food could potentially lead to heart disease?
I am not cross fit certified but as part of my degree I was required to take sever nutrition classes. As a personal trainer it should be your responsibility to know not only know the exercise aspects that contribute to an overall healthy lifestyle but also the dietary aspects.
Giving food recommendations is a must. What if your client comes into a session having not eaten anything since the day before? What if they decided to eat only a protein bar for breakfast? The will have depleted they glycogen stores during the night and thus will have little to no energy to dedicate to their workout.