Final answer:
The RN should instruct the UAP to 'Brush the client's teeth and gums gently using a soft toothbrush.' This prevents further irritation to the inflamed tissues of the mouth cause by stomatitis.
Step-by-step explanation:
The correct instruction that a registered nurse (RN) should give to unlicensed assistive personnel (UAP) for the oral care of a client with stomatitis is "Brush the client's teeth and gums gently using a soft toothbrush." This is because stomatitis involves inflammation of the mouth, which makes the mouth more susceptible to injury. Using a soft toothbrush can help in providing oral care without causing additional trauma to the sensitive oral tissues.
An alcohol-based mouthwash should be avoided as it can cause irritation and drying, exacerbating the condition. Similarly, providing a spicy meal after oral care should also be avoided as it can irritate the inflamed oral tissues. While applying petroleum jelly to the client's lips may prevent dryness and cracking, it is the gentle brushing that is critical for oral care in this scenario.