How does a Muslim marry a Christian woman if she was married to a Christian man and then left him, if Islam is a condition for the validity of divorce?

The notion that divorce is not valid from a non-Muslim is not absolute. A disbeliever divorces a disbelieving woman, and his divorce is valid and recognized.

If a wife embraces Islam while married to a disbelieving man, she becomes unlawful to him, and they are separated. Her completion of the waiting period (iddah) is observed, based on Allah’s Almighty saying: ﴿So if you ascertain that they are believers, then do not return them to the disbelievers; they are not lawful for them, nor are they lawful for them.﴾ However, if he embraces Islam before the expiration of her waiting period, she is returned to him.

Perhaps the phrase "divorce is not valid from a non-Muslim" refers to the issue of judicial rulings, specifically whether a non-Muslim judge can divorce a wife from her husband. A disbeliever has no authority over a Muslim. Therefore, Islamic centers act in place of Sharia courts when they are unavailable. If a woman obtains a civil divorce, she should then approach the Islamic centers, where qualified scholars in these matters can finalize the process from an Islamic perspective.