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Firstly:
Free mixing in workplaces, schools, universities and elsewhere is haraam, because of the evils to which it leads. This has been discussed previously in the answers to questions no. 1200 and 50398.
Secondly:
You were mistaken in your actions when you listened to this young man when he said that he liked the girl, and you made a mistake by telling her, because the effects and consequences of this kind of love in a mixed atmosphere are well known.
What you have to do is repent to Allah, may He be exalted, and advise the girl to put an end to this haraam relationship and to look for work in a place in which there is no mixing.
And Allah knows best.