The theologian was right, at least on this point. As far as we know*, Paul did only quote from the LXX, not from the Hebrew texts. This may have been because as a diaspora Jew he was not sufficiently familiar with the Hebrew language (in spite of Acts 22:3) or so that his gentile converts could read his sources in Greek.
Christopher D. Stanley says in As it is Written, page 3 (edited by Porter and Stanley) that the wording of his quotations and allusions often diverges significantly from the texts that he cites. The sense that he derives from the biblical text also deviates fairly often from what modern readers might see as the "original meaning" of the passages to which he refers. Similar problems can be discerned in the writings of other New Testament authors.
The four New Testament gospels contain numerous quotations and allusions to the Old Testament scriptures. As with Paul's epistles, all the gospels were originally written in Greek (including Matthew, in spite of a long-held tradition) and when they refer to the scriptures, these references are identifiably to the Septuagint (LXX).
(*) In 1 Corinthians 3:19, Paul says, "... For it is written, He taketh the wise in their own craftiness," but there is no known text that corresponds closely to this. Steve Moyise say, in 'Quotations', published in As it is Written, page 16 (edited by Porter and Stanley) that the thought is close to Job 5:12-13, but the LXX that comes down to us is very different to Paul's words. Moyise says it is possible that Paul is offering his own translation of the Hebrew, but this is a minority position today.