This is a good example of why you should make sure everyone involved in a presentation is on the same page before you begin. If this person saw a flaw, it should have been identified and addressed much earlier. I would say that it was the controller’s responsibility to say something to you sooner. Ethically, your job at that point is to tell the investors that you will investigate, and recalculate if there turns out to be a problem. However the controller needs to learn some business ethics too. Offer transparency (making sure you have honest sharing of numbers).
If you knew about the problem already and tried to hush it up, that’s different. Hushing it up by trying to keep the controller quiet would have been unethical, and the controller would basically be a whistle-blower in this situation. Your only response would be to kill the deal.