A little more context regarding the background of her parents would be helpful.
1. For example, are her parents open theists? The author of the post comments towards the bottom:
"The best philosophical and biblical option in this regard seems to be Open Theism to me."
2. Are they Protestants? If so, then are the positions of theologians in church history the "mic drop" standard according to which you should be discussing with them?
3. Do they, like the author, deny the doctrine of original sin? If so, on what just grounds do they think infants die? In particular, do they think newborn infants or infants in the womb freely choose to sin? If so, see
Romans 9:11.
4. Augustine was a brilliant systematician, but he needn't be defended on all points. He denied the perseverance of the saints, for example, while coherently retaining sola gratia (
link).
Nevertheless, vague assertions that Augustine's thought on the subject of man's will is suspect due to his former position of Manichaeism is a genetic fallacy. The author even acknowledges:
To suppose any later reticence by Augustine to attribute a large degree of freedom to the will in Adam's progeny implies Augustine reverted to Manichaeism is an assertion in search of an argument - an argument you won't find in that article nor in its quotes of Julian, Pretyman, et al..
5. Augustine did not dichotomize volition and necessity. For example, see Augustine's
Against Two Letters of the Pelagians (Book I, Chapter 7):
That is, there is always the question of semantics. What is choice? And what is
free choice?