I hope you don't have to, but If you must to do it for the sake of the family, teach the student about the Bible and nothing more! Help the student to investigate and learn for himself. That will be enough.
On a related note: Man. That sucks. I don't even know what to say. But here is an idea: Do you have family study? Maybe that can help your wife. Do a serious study with your wife about, for instance... the Generation changes (Mat 24:34). Tell her to prepare for that. She can do her research and you can do yours. Use only WT literature. Let her comment about here findings. And ask her what are her conclusions.
You can read with her 2 Cor. 11. That in part says:
3 But I am afraid that somehow, as the serpent seduced Eve by its cunning, YOUR minds might be corrupted away from the sincerity and the chastity that are due the Christ. 4 For, as it is, if someone comes and preaches a Jesus other than the one we preached, or YOU receive a spirit other than what YOU received, or good news other than what YOU accepted, YOU easily put up [with him].
13 For such men are false apostles, deceitful workers, transforming themselves into apostles of Christ. 14 And no wonder, for Satan himself keeps transforming himself into an angel of light.
This portion of the Bible basically says: We may have accepted corrupted information different than what the Bible says. If that information is not in the Bible, but it is just a interpretation, it could be just a human interpretation with no value (a simple theory) or even worse it can come from Satan himself and his "light".
You can add: "If the WT don't know what that text about the generation means, how can be sure the interpretation of the rest of the chapter is correct".
Last advice: Keep the topic. Any other topics can be added in a list for the next family study. If she wants you to teach somebody else, she should let you study with her too.
Make sense? I hope it helps.
-ootb