"For many years Jehoshaphat lived in peace, unmolested by surrounding nations. “The fear of the Lord fell upon all the kingdoms of the lands that were round about Judah.” Verse 10 . From Philistia he received tribute money and presents; from Arabia, large flocks of sheep and goats. “Jehoshaphat waxed great exceedingly; and he built in Judah castles, and cities of stores.... Men of war, mighty men of valor, ... waited on the king, beside those whom the king put in the fenced cities throughout all Judah.” Verses 12-19 . Blessed abundantly with “riches and honor,” he was enabled to wield a mighty influence for truth and righteousness. 2 Chronicles 18:1 . PK 192.2"
"Now Jehoshaphat had riches and honour in abundance, and joined affinity with Ahab."
Ellen White directly references 2 Chronicles 18:1 in this passage.
"Some years after coming to the throne, Jehoshaphat, now in the height of his prosperity, consented to the marriage of his son, Jehoram, to Athaliah, daughter of Ahab and Jezebel. By this union there was formed between the kingdoms of Judah and Israel an alliance which was not in the order of God and which in a time of crisis brought disaster to the king and to many of his subjects. PK 192.3"
"Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son."
The EGW passage describes a marriage alliance that violated the principle outlined in Deuteronomy 7:3 against intermarriage with those who do not follow God. This is a clear application of the principle.