All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We hope that you are not sinful for helping this girl enrol in that mixed university, if at that time you did not know that it was forbidden to enrol in it, and you did not do so with the intention of guiding her to sin. Allaah Says (what means): {Our Lord! Punish us not if we forget or fall into error.} [Quran 2:286]. It is confirmed in a narration reported by Imaam Muslim that Allaah said in a Qudsi narration: "Indeed, I honoured their supplication (fulfilled it)."
Now, you may do the two following things:
1- If there is another alternative (another university) where there is no mixing between boys and girls, in this case you are obliged to advise her to leave this university and move to the second university where there is no mixing, because there are many disadvantages in co-education universities; for more benefit, please refer to Fatwa 84281.
2- If there is no alternative, and she is in need for these studies, then you must remind her to earnestly fear Allaah and avoid mixing with men or talking to them without a need. In addition to this, her presence in the university should be according to the need only.
For more benefit, please refer to Fatwa 87310.
Allaah Knows best.