I'm sympathetic to this - I am getting into pretty formal debates about the subject myself. Here's a few things that might help (I assume you'll argue for the research?):
If they take a moral principle stance: If they argue that it is wrong because it is a living human being, or that in some way it has special 'living' standing, I would say first that they confuse what 'living human' means: by their definition, a human arm hooked up to a machine constitutes the same kind of life - that is clearly not a legitimate thing to say; a human arm does not deserve any protection from the law, does it? no. An embryo possesses none of the qualities that make us deserving of rights, responsible for our duties, and it is certainly not capable of decision making, which is the only way someone can be considered responsible at all (without that faculty, a human is just another animal, same as a fish. No decisions, no responsibility, no special status). This usually leads someone to the next type of argument:
If they argue about 'potentiality': some people like to argue that an embryo deserves protection because it has the potential to become a rational, human being. This is a poor argument because a thing's potential does not change what it actually is at the moment. A seed has the potential to turn into a tree, but is still merely a seed. This is why nobody cries when we destroy a seed, but - rightly - cry out when trees are going to be cut down. Although the two are connected, they are still distinct beings subject to their individual characteristics: in the case of a human person, he/she is rational, capable of decision making, responsible (inherently so), and accountable. An embryo is such a basic organic thing much too abstract to defend the rights of.
If I were finishing off the debate, I would do so with the following: "We need to first establish what makes any human special in the first place. This is usually done through appeals to rationality, or personhood: this is the crux. To be a PERSON and not simply a human is what makes us deserving of rights and duties. All the complexity of persons - their thoughts, loves, desires and beliefs, and their flaws - is what makes them special and we should defend that. In present times what is needed most are solutions to the problems that are bearing down on the 6 billion persons we share the world with. We need to support justice, equality, and prosperity and desperately need to find a solution to the abhorrent suffering that we see every night on the news. What matters are people, not a distant abstract organism that can only rely on its biology - which is not enough."
Sorry for the rant, but I hope it helped getting the juices flowing. Good luck with the debate.
