Question: A non-Muslim asked ""Why can't a supposedly perfect God simply forgive?" How should he be answered?
Answer: Before giving a template for how to answer such a question, I think it is important to point out that such questions are not asked in a genuine quest for knowledge, but out of arrogance and in order to mock and distract. The sillyness of the question is an indicator of this.
I would simply answer, "He forgives if He wills to do so. Noone and nothing can prevent Him from forgiving or punishing as He wills, and noone and nothing can change what He has willed". Based on the response you can know where to take it next. Do not let yourself be manipulated, and do not engage in a debate unless you are knowledgeable enough to win.
... and Allah knows best.