Is a young man forbidden from marrying all of his maternal aunt's daughters, or only the middle one, given that he was breastfed by the middle one five satisfying times or more?

What is established by lineage is also established by fosterage. So, if your friend was nursed by your aunt's mother along with your aunt's daughter, he becomes a brother to all of your aunt's daughters. However, if he was nursed with your aunt's daughter by a woman who is a stranger (not your aunt's mother), then the prohibition (of marriage) applies only to the one he was nursed with. It is a condition for fosterage to be legally prohibitive (of marriage) that there be five full, satisfying breastfeeds.