Is it permissible for the questioner's son to marry the daughter of her friend, who was nursed by the questioner twice on the same day?

Scholars differ on the amount of suckling that establishes a prohibitive relationship, and the preponderant opinion is that less than five sucklings do not establish prohibition. It is permissible for your son to marry the girl who suckled from you twice, though it is more cautious to refrain, as a way of avoiding disagreement.