Final answer:
Encouraging the client to perform as much self-care as possible is the best intervention to prevent further regression in the client's personal hygiene.
Step-by-step explanation:
The best intervention to prevent further regression in the client's personal hygiene would be encouraging the client to perform as much self-care as possible. This approach promotes independence and autonomy, which can help maintain the client's dignity and sense of self. By providing support and assistance as needed, the nurse can empower the client to engage in personal hygiene activities to the best of their ability.