Or HaChaim on Exodus 18:8/אור החיים על שמות י״ח:ח׳

1 ויספר משה לחתנו, Moses told his father-in-law, etc. Although the Torah told us that Yitro had heard all that G'd had done, there may have been some details which he had not heard about. Alternatively, Yitro had assumed that what he had heard previously was grossly exaggerated; Moses now explained to him that what he heard was true to the last detail. It is also possible that until Moses told him otherwise Yitro had been under the impression that the Israelites were still vassals of the Egyptians, not totally free. Moses told him of the slaying of the guardian angel of the Egyptians, something that Yitro could not have heard about from any other source as only the Israelites had seen the "dead" guardian angel (Zohar second volume page 52). Once Yitro heard about this he knew that the Egyptians did no longer have any hold over the Israelites.

2Perhaps the Torah itself alludes to this with the words את כל התלאה אשר מצאתם בדרך ויצילם השם, "all the travail that had come upon them on the way from which G'd had saved them." Seeing Yitro had been a religious leader he would be familiar with the fact that different nations had guardian angels in the celestial regions; Moses told Yitro about the personal pursuit by that guardian angel and how the Israelites had been very frightened of that guardian angel. When Yitro heard about all this he exclaimed: "blessed be the Lord who has delivered this nation from the hand of Mitzrayim (the guardian angel)."