That atheist had no clue what he was talking about. (No marvel, right?)
Then the priest shall charge the woman with an oath of cursing, and the priest shall say unto the woman, The LORD make thee a curse and an oath among thy people, when the LORD doth make thy thigh to rot, and thy belly to swell; And this water that causeth the curse shall go into thy bowels, to make thy belly to swell, and thy thigh to rot: And the woman shall say, Amen, amen.
-Num 5:21-22
If the woman was caught in fornication/adultery, or rather, she was caught, meaning that there was evidence against her, as perhaps her husband may have known about it, but there was no direct eye witness and no one had caught her in the act (v13) and she lied about it, this was a method to determine if she lied. If she was innocent, and spoke the truth, then no harm would come to her, but if she lied before God, the curse of rotting thighs, which would leave a personal unable to walk, and a swelling belly, which, outside of pregnancy, is dangerous to one's health, especially if this is referring to a particular illness God is cursing her with, and it certain is an outward shame to a woman, which causes her great embarrassment. Either way, she was to say "Amen," that is, "Truly" or "This is the truth" to whichever punishment or blessing she received from the Lord.
There is nothing even remotely close to abortion talked about in this passage. The "belly swelling" is not pregnancy; she is not yet with child. This is proven by verse 28:
And if the woman be not defiled, but be clean; then she shall be free, and shall conceive seed.
-Num 5:28
Meaning, she would conceive afterwards, being blessed by God in cleanness.