Is there any prohibition (Torah or rabbinical) against a Noahide man having relations or even marrying a Zera Yisrael woman who is not halachically Jewish?
For the hypothetical case here, the woman was raised in a reform community, and her grandmother's conversion was likely halachically invalid. If the grandmother does not have a valid conversion, then her daughters and granddaughters would be Zera Yisrael and not halachically Jewish.
Therefore the question then arises, is a Zera Yisrael woman permitted to a Noahide man?
Also, what level of due diligence is a Noahide required to perform when confirming a woman's claim that her grandmothers conversion was not valid? In the mouths of two or more let a thing be established?