What should I do after the fiancé of the girl I was speaking with asked me to testify before his and her family that she knew me and used to meet me?

The questioner must sincerely repent for the illicit conversations that took place between him and the girl. If the girl is religious and of good character and has repented, then he should proceed with marrying her. Otherwise, he has the right to annul the engagement in a way that does not contradict Islamic law. He should not bear witness to scandal; rather, he should conceal it for himself and for others. If he is compelled to speak about what happened, then he should use allusive speech (ma'arid).