וַיֹּאמֶר חֹתֵן מֹשֶׁה אֵלָיו לֹא־טוֹב הַדָּבָר אֲשֶׁר אַתָּה עֹשֶׂה׃
But Moses’ father-in-law said to him, “The thing you are doing is not right"; (Shmot 18)
The Halachah says (Rambam T"T 5,9):
רָאָה רַבּוֹ עוֹבֵר עַל דִּבְרֵי תּוֹרָה אוֹמֵר לוֹ לִמַּדְתָּנוּ רַבֵּנוּ כָּךְ וְכָךְ. וְכָל זְמַן שֶׁמַּזְכִּיר שְׁמוּעָה בְּפָנָיו אוֹמֵר לוֹ כָּךְ לִמַּדְתָּנוּ רַבֵּנוּ.
If one beholds his master transgressing the words of the Torah, he should say to him: "Our master, you have instructed us such and thus." Whenever he quotes a lecture in the presence of his master, he should say: "So have you, our master, instructed us."
Moses just descended from 80 days of continuous learning Torah from G-d Himself, presumably knowing the whole Torah.
Moses didn't teach the Torah yet to the masses.
A yesterday idolatry pries that worshipped all the idols available and a freshly converted old man judges Moses' behavior publicly.
The [quality of the] advice by itself is not a problem but its phrasing seems bad. He should have said, "Moses, please reveal the laws of the Jewish Judicial system that G-d commanded upon you. Is this how G-d commanded you?" But he was sure Moses is wrong, but how dared he?
As the Gemmorah in Kiddushin 30 explains that respecting Talmidei Chachamim equals (and stems from) respecting G-d and His Torah, it is clear to me that Yitro comes not against Moses as a person but against the whole Torah and its scholars.
How Halachic is it, and if not, who criticizes Yitro for this conduct?
Please note, I don't discuss the advice itself here, only the way of referring to Moses.