Does a divorce, which a suitor made conditional on his fiancée's admission of a previous relationship, take effect, and what is your advice to this girl who regrets the past and wants to open a new page with her husband?

What the husband does by asking and insisting about the life before marriage is not permissible. It is a violation of Allah's saying: (And do not spy), and the Prophet's (peace be upon him) saying: (The extremists are ruined).

The divorce has already occurred by his linking it to a lie, as long as the lie has taken place. He must be informed of this, and that it is not permissible for him to investigate the past. He should either hold onto her with kindness, or separate with goodness. It is not permissible for him to hold onto her while mistreating her in this manner.