My husband wants to know if the 70,000 killed by a plague by an angel knew there was a God?
You are referring to those who died as a result of David's sin in numbering the men of war in Israel. "So the LORD sent a plague upon Israel from the morning till the appointed time. From Dan to Beersheba seventy thousand men of the people died. And when the angel stretched out His hand over Jerusalem to destroy it, the LORD relented from the destruction, and said to the angel who was destroying the people, "It is enough; now restrain your hand." And the angel of the LORD was by the threshing floor of Araunah the Jebusite" (II Samuel 24:15-16). The plague not only punished David, but it also punished the nation for allowing David to number them, and it ruined David's count.
Those who died were Israelites. Since it was the Law's requirement for all Israelites to be taught of God, it would be reasonable to assume they knew there was a God and what God required of them. "You shall teach them diligently to your children, and shall talk of them when you sit in your house, when you walk by the way, when you lie down, and when you rise up" (Deuteronomy 6:7).