Your friend is right that there is no biblical prohibition rather only a Rabbinical prohibition for premarital relations and the Christian understanding is based on the Jewish understanding.
לֹא־תִהְיֶ֥ה קְדֵשָׁ֖ה מִבְּנ֣וֹת יִשְׂרָאֵ֑ל וְלֹֽא־יִהְיֶ֥ה קָדֵ֖שׁ מִבְּנֵ֥י יִשְׂרָאֵל׃
according to Halacha translates Like Targum Onkles: "A woman should not marry a canaanite slave (who's accepted mitzvos as he's had Tevila and mila see Yevamos 46a) and a man should likewise not marry a Canaanite slavewoman (who's accepted mitzvos as she's had Tevila see Yevamos 46a)." (This is the reason why a Mamzer can marry a slave woman as he is already a product of an illegitimate relationship see Kiddushin 59a, see Rashi there, and Rambam Issurei bia 15,3. and Shulchan aruch Even haezer 4,20.)
The Rambam in Hilchos Naara 2,17 says:
חייבה תורה לאונס ולמפתה ממון ולא מלקות בשאירע הדבר מקרה שלא מדעת אביה ולא הכינה עצמה לכך שדבר זה אינו הווה תמיד ואינו מצוי. אבל אם הניח בתו הבתולה מוכנת לכל מי שיבוא עליה גורם שתמלא הארץ זמה ונמצא האב נושא בתו והאח נושא אחותו שאם תתעבר ותלד לא יודע בן מי הוא והמכין בתו לכך הרי היא קדשה ולוקה הבועל והנבעלת משום לא תהיה קדשה-
The Torah demands monetary compensation when relations were against her will or with her consent but against her fathers will (her father has dominion over her till she is about 12 and a half). If she consents (against her fathers will) to relations by chance i.e to 1 person (e.g she has a boy friend) they do not get Malkus from the Torah (But of course there is a Rabbinical prohibition of Kallo Belo brocho assura lebaala kenida in Masechet כלה 1,1.) There is only a Torah prohibition which is chayav Malkus (lashes) if she or her father makes herself into a harlot who is available to many men...
A Torah prohibition is therefore not applicable if it is a casual relations with a boyfriend not intended for the sake of prostitution.
Accoring to the Kesef Mishna (ibid) this prohibition of prostitution for many men is based on the sifri and the Raavad agrees with the Rambam that there is a Biblical prohibition for a prostitute available many men but not to 1 man by chance.
However the Ramban in Hasogos sefer hamitzvos lo saase 355 holds even prostitution is only rabbinically forbidden (and premarital relations with one man is definitely not prohibited from the Torah) because prostitution is only prohibitted mideoraisa (biblically) to a gentile, canaanite slave/slavewoman, ervah,yisroel to mamzer(es), cohein to divorcee cohein Gadol to widow, petzua dakka and crus shophcho with a yisraelis.
Based on this Every opinion holds that there is no prohibition from the Torah on premarital relations for which the woman is planning on marrying that man and she is not available to other men, but there is a rabbinical prohibition*.
However a Nidda (woman who has a menstrual cycle and not immersed in a mikva) is forbidden Min Hatorah.This applies to a regular unmarried woman is a nidda unless she has immersed to a "mikva" (ritual bath) to purify herself and is forbidden from the Torah (Leviticus 18,19).
*Note:Those who think rabbinical prohibitions are not serious are sinners who will be stumbled by the Torah ופושעים יכשלו בם