Am I sinful for causing an illicit love relationship between a young man and a girl in the workplace, with the relationship continuing despite failed attempts at marriage?

Mixing between genders in workplaces, schools, universities, and other settings is forbidden due to the corrupting influences that result from it. You erred in listening to this man and informing the girl of his love. Your duty is to repent to Allah, advise the girl to sever the forbidden relationship, and seek employment free from gender mixing.