Is the questioner considered a slanderer and unjust to the woman her husband was speaking with, after she falsely told her husband that the woman said she had known him for a long time and that they had met and exchanged photos?

What you told your husband about the woman is not slander that necessitates the prescribed punishment (hadd), but it is a lie and an injustice to her. Therefore, you must repent to Allah and inform your husband that you lied to him, and seek forgiveness for the woman. Ibn al-Qayyim said that the repentance of one who backbit or slandered is between him and Allah, by mentioning the backbitten or slandered person with the opposite of what he mentioned about them, and by seeking forgiveness for them. You must also advise your husband to stay away from this woman. And spying is forbidden.