What is the ruling in Islam regarding a fiancée informing her suitor about a previous emotional relationship, which may have included moral transgressions, if he asks about it? And is this considered concealment or withholding of testimony?

It is permissible for someone who is consulted regarding a marriage proposal for a woman to mention her defects that affect the marriage, because this constitutes advice. They cite as evidence what the Prophet, peace and blessings be upon him, said to Fatimah bint Qays concerning Mu'awiyah and Abu al-Jahm. It is stipulated that there must be an urgent need to mention the defects, and that the advisor should limit themselves to defects that compromise the desired benefit from the marriage. If the woman has repented sincerely from a previous relationship, then she should be shielded. However, if she has not repented, then her situation must be disclosed to the suitor.