Is the girl's first marriage, which took place without a guardian and with three witnesses (one of whom was on the phone), Islamically valid? And is her second marriage to her current suitor also Islamically valid, especially since she is pregnant from the first marriage and concealing her pregnancy?

This problem has two aspects: jurisprudential and educational. From a jurisprudential perspective: the marriage of a woman without a wali (guardian) is invalid according to the majority of scholars, and any child born from such a union is attributed to his father due to the presence of shubhah (doubt/ شبهة). It is not permissible for a wife to marry another man before her first marriage is annulled, nor is it permissible for a man to marry a pregnant woman until she gives birth. Your friend's second marriage is invalid, and she must repent and rectify the error by being frank with her family. It is not permissible to force a girl to marry someone she does not desire. From an educational perspective: fearing Allah, frankness, and honesty are the keys to solving family problems, and true love for building a stable family is that which observes Allah's law.