Final answer:
To avoid potentially dangerous side effects, the nurse should tell the patient to avoid salt substitutions in his diet while taking the ACE inhibitor.
Step-by-step explanation:
The nurse should tell the patient to avoid salt substitutions in his diet. ACE inhibitors can increase the level of potassium in the bloodstream, and salt substitutions often contain potassium chloride. Consuming large amounts of potassium while taking an ACE inhibitor can lead to a condition called hyperkalemia, which can be dangerous for the patient.