Who is to ask a fornicatress to repent, and how is repentance to be sought, and is he who informs his parents of the fornication of a girl he wishes to marry sinful?

We do not know what exactly is meant by "istitaabah" (seeking repentance).

If what is meant by it is asking her to repent, then this is guaranteed for anyone who knows of her sin, and it falls under the category of inviting to Allah and enjoining good.

However, if what is meant is the jurisprudential term, then "istitaabah" does not exist in the context of zina (adultery/fornication), and the execution of the prescribed punishment (had) does not depend on it. Rather, the had is carried out on the fornicator/adulterer if the crime is proven, even if they repent.

It is not permissible to contract a marriage with a fornicatress/adulteress except after her repentance, based on the verse: "The fornicator does not marry except a [female] fornicator or a polytheist, and none marries a [female] fornicator except a fornicator or a polytheist, and that has been forbidden to the believers" [An-Nur: 3].

It is obligatory for anyone who wishes to marry her after her repentance to conceal her disobedience and not inform anyone about it.