It is not permissible for you to say that you were present at a lecture from which you were absent, because this is lying. The supervisor has no right to allow you to do that, because that is cooperating in sin and betraying a trust. Allaah says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
And Allaah knows best.