Final answer:
The client is susceptible to illness due to effects of stress on the immune system.
Step-by-step explanation:
The best rationale for the therapist's advice to encourage open discussion of feelings, proper nutrition, and exercise is that the client is susceptible to illness due to effects of stress on the immune system. Grieving the loss of a spouse of 34 years can cause immense emotional and psychological stress, which can weaken the immune system. By addressing the client's feelings and promoting healthy lifestyle practices, such as good nutrition and exercise, the therapist aims to support the client's overall well-being and reduce the risk of illness.