What is the ruling on a man marrying a girl whose sister was suckled by the girl's mother for three days?

Allah Almighty specified the women forbidden for marriage in the Holy Quran. The verse states: "Forbidden to you are your mothers, your daughters..." then "And lawful for you are what is beyond those." Based on this, your friend is permitted to marry the neighbor's daughter because she is not his foster sister, but merely his sister's sister. She falls under the general permissibility of "And lawful for you are what is beyond those," and there is no specific text that excludes her from this general ruling.