Final answer:
The sentence "she was having her dinner" can be changed to passive voice as "her dinner was being had."
Step-by-step explanation:
In this sentence, the action of having dinner is being done to the subject, making it a passive construction. To change the sentence to passive voice, we can say, "Her dinner was being had." Here, the subject (she) is now the receiver of the action, and the verb (had) is in the past tense.