Final answer:
The nurse should advise the client to use a soft toothbrush for oral care to help with stomatitis caused by radiation therapy to the neck.
Step-by-step explanation:
The nurse should include the intervention of advising the client to use a soft toothbrush for oral care. Stomatitis, which is inflammation of the mouth, is a common side effect of radiation therapy. Using a soft toothbrush can help prevent further irritation and promote healing of the mouth.