Is their marriage considered valid, are their actions considered adultery, and will God forgive them if they repent?

What occurred between the man and the woman is adultery and a mockery of the Sharia. It is a void marriage, without a guardian or witnesses. They must repent to Allah for this grave sin, sever their relationship, andDistance themselves from temptations. They must enter into a valid, lawful marriage after repenting from the adultery they committed.