Also, one thing to be aware of is that her opponent is almost certainly going to say at some point that homosexual “marriage” is a “right.” This needs to be challenged and firmly, especially because gay “marriage” proponents are probably disproportionately likely to be atheist liberals who deny all the metaphysical bases from which “rights” could be said to arise.
So, if confronted with this claim, she needs to ask, and vigorously demand an answer to, these questions: what right? Where does this right come from – what is it’s source? Why is it no one noticed it existed until approx. 5 minutes ago?
Here it’s worth looking at the historical consensus about where rights come from. For far-rightists like me, rights derive from our duties. This is the natural law take. A right is the “can” to duty’s “ought.” But natural law furnishes absolutely no basis for a “right” to gay marriage, precisely because you have a duty to avoid that which goes against the natural law and therefore a duty not to have gay sex entirely. So clearly, this “right” to gay “marriage” doesn’t come from natural law.
For center-rightists, including most of Protestantism, “rights” come from God; they are revealed by Him directly or else derive from the duties He has proscribed for us. Here, too, there is no basis for saying there is a “right” to gay marriage, especially if her opponent is either an atheist (doesn’t believe in God) or a secularist (doesn’t believe God should have anything to do with the ordering of civil society). God, at least the God of the Bible, has clearly and repeatedly condemned sexual immorality of all sort, including punishing it with fire and ordering His subjects to punish it (e.g., the specifically-mentioned killing of Canaanite temple prostitutes). The only form of sexual arrangement we know with certainty that God has endorsed is heteronormative marriage, and as St. Paul suggests, this was only a grudging concession. So, clearly no “right” to gay marriage comes from God, either.
I have heard it said before that this “right” comes from the “innate dignity” of the human person. I would defy that person to explain what this “innate dignity” is, what it means, and how/why a “right” to “gay marriage” derives from it. Incidentally, the person I heard this from was a leftist atheist who had just denied my natural law take against homosexuality on the grounds that “human nature” is a subjective fiction, that the phrase “humanity” is a mere heuristic for describing a large mass of dissimilar people with no correspondence to objective reality. Well, obviously, if he believes this, there is no reason to believe in anything like the “innate dignity” of the human person, anyway. You can derive an understanding of “innate dignity” from a Christian or Aristotelian understanding of human nature, but both of these systems explicitly deride homosexuality as contrary to man’s dignity. Man’s dignity consists in him doing that which is consistent with his nature, i.e., in becoming an excellent, exemplary human. This is clearly inconsistent with homosexuality in general. You certainly can’t derive such an understanding from a system that denies there is such a thing as “humanity” or “human nature,” much less “God.”
Finally, there’s the “social contract” approach. Rights derive from social consensus. Obviously, though, they don’t believe this, or they would never have started their push for gay “marriage,” since support for was in the single-digits when they started demanding that this “right” be recognized. You can’t claim the social contact yields a right when trying to renegotiate the social contract to make up that right in the first place. The best that can be said is that they want to renegotiate the social contract because they think it would produce a better arrangement of things than the current arrangement. In that case, her opponent needs to stop talking about “rights” entirely and start talking about whether or not gay “marriage” is good or bad. Insisting that it’s a “right,” in that case, is tantamount to mutilating the language in order to endow his/her position with a moral normativity it simply doesn’t warrant.
Once her opponent is forced into this position, she can start asking what, exactly, constitutes “good” and “bad.” Here, she must point out, we cannot divorce our discussion from what is moral, since morality is, by definition, a coherent understanding of “good” and “bad.” With some finesse she can compel her opponent to admit that she is a consequentialist and utilitarian, i.e., she wants gay “marriage” because her opponent thinks happiness is the highest good and doesn’t care what means are used to achieve an increase in happiness. Here, she can simply point out the horrors that would arise from such a system, rigorously applied. For instance, it would furnish no basis for objecting to a doctor murdering one patient and harvesting his organs in order to transplant them into five others, saving their lives. Such a thing would, indeed, produce an increase an happiness, yet no one thinks it would be desirable. On the contrary such an act would be a moral abomination and such a system which endorsed that kind of behavior would be utterly reprehensible. Once she’s got her opponent backed into this corner, she’s basically won the debate.
I hope this helps. I think it’s really cool you’re supporting your daughter and helping her to prepare for her debate this way.