Maybe HD should buy non-refundable tickets, but I think that is a rather passive aggressive stance to take. The better, stronger response would be to just confront her in a calm manner and tell her he planned this long ago and that he is going. End of story. That gives credibility to his determination and his boundary.
Saying that he won't fall for whatever excuse she throws at him because he has non-refundable tickets is really hiding from that confrontation and I think it leaves the door open for her to challenge him again.
