I breastfed my friend’s daughter for several weeks and she is my foster suckling daughter. She is now 17 and my son is 18. I understand they could NEVER get married as they shared the same milk.
Is it permissible for my 22 year old son to marry her? Or is that Haram for them?

The breastfeeding which establishes the relationship of mahram is that which takes place five or more times in the first two years. If it was five full breast feedings within the first two years, then she became your foster suckling daughter and your son who you breastfed with her, becomes her brother through breastfeeding, and ALL your children, both males and females, become siblings to her through breastfeeding; thus it is proven that the relationship of mahram exists between ALL of them.

The basic principle concerning that is the verse in which Allah says (interpretation of the meaning):
“Forbidden to you (for marriage) are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters.” [al-Nisa’ 4:23].

Prophet Muhammad (blessings and peace of Allah be upon him) said: “What becomes mahram (forbidden for marriage) through breastfeeding is that which becomes mahram through blood ties.” Narrated by al-Bukhaari, 2645; Muslim, 1447.

So none of your children can marry your foster suckling daughter EVER as they are all considered sibling to each other.

Allah Knows Best