We see the same problem when we consider the "clean" animals, which Noah included in his ark (Gen 7:8) intended for burnt sacrifices to the Lord (Gen 8:20). That is, the number of "clean" animals were tripled in the ark for the purposes of burnt sacrifice (Gen 7:2). How did Noah come to discriminate "clean" and "unclean" animals in the absence of the explicit Law of Moses? Another problem is the destruction of Sodom and Gomorrah. The written law against homosexuality (Lev 18:1-30) was non-existent at the time, but Sodom and Gomorrah and their suburbs appear later in the Hebrew Bible and Christian New Testament as "immoral" cities. How were they "decadent" when there is no written law to define what was "immoral"?
So, in the absence of any written law, to what was Jethro referring, when he mentioned "statutes and laws" to Moses (before the giving of the Law at Sinai)?
Let's start with the Prophet Ezekiel, who provides the "legal reason" why Sodom and Gomorrah and their suburbs were destroyed.
Ezekiel 16:46-50 (NASB)
46 Now your older sister is Samaria, who lives north of you with her daughters; and your younger sister, who lives south of you, is Sodom with her daughters. 47 Yet you have not merely walked in their ways or done according to their abominations; but, as if that were too little, you acted more corruptly in all your conduct than they. 48 As I live,” declares the Lord God, “Sodom, your sister and her daughters have not done as you and your daughters have done. 49 Behold, this was the guilt of your sister Sodom: she and her daughters had arrogance, abundant food and careless ease, but she did not help the poor and needy. 50 Thus they were haughty and committed abominations before Me. Therefore I removed them when I saw it.
Sodom and her sisters (or "sister cities" in the area) perished because they loved themselves more than they loved their neighbors. (The definition of "neighbor" encapsulates the most helpless and weak in society, which are widows and orphans.) The Sodomites had abundant food and careless ease, but they did nothing to address the needs of the widows and orphans. Lot was a resident of Sodom, and the Sodomites accosted him to hand-over the angels for sex (Gen 19:4-5). Thus "Sodom and her sisters" perished because of their arrogance and haughtiness, which was expressed through moral recklessness.
Thus the oral "proto"-law in the Hebrew Bible was to love ones neighbor as oneself, which is what the Sodomites failed to do. When Jesus described the parable of the Good Samaritan, he amplified our understanding of this principle. That is, the "proto"-law is not defined by what we think what loving-our-neighbor means, but what our neighbor thinks loving-our-neighbor means. In other words, the reality of loving ones neighbor as oneself is perceived not by what we perceive what we do to our neighbor (through our love), but by what our neighbors perceive we are doing to them (through our love).
Thus while the Sodomites considered their lifestyle normal, that normality was not "seen" by Lot (or the angels). The behavior of the Sodomites was therefore abominable because their arrogance and haughtiness resulted in active aggression (sexual assault) and passive aggression (neglect of widows and orphans). As the defender of the widow and orphan, the Lord therefore executed justice.
Deuteronomy 10:17-19 (NASB)
17 For the Lord your God is the God of gods and the Lord of lords, the great, the mighty, and the awesome God who does not show partiality nor take a bribe. 18 He executes justice for the orphan and the widow, and shows His love for the alien by giving him food and clothing. 19 So show your love for the alien, for you were aliens in the land of Egypt.
The visitors to Sodom (angels) were the "aliens," for whom the Sodomites showed no love in the manner that the angels would have "seen" or perceived that love.
Another excellent example in the Hebrew Bible is Nineveh. When Jonah went and preached repentance to them, there was no mention of any violation of obedience to YHWH (which would suggest covenant relationship) but to "God." The king of Nineveh stated the following:
Jonah 3:7-9 (NASB)
7 He issued a proclamation and it said, “In Nineveh by the decree of the king and his nobles: Do not let man, beast, herd, or flock taste a thing. Do not let them eat or drink water. 8 But both man and beast must be covered with sackcloth; and let men call on God earnestly that each may turn from his wicked way and from the violence which is in his hands. 9 Who knows, God may turn and relent and withdraw His burning anger so that we will not perish.”
The relationship of the Lord God to the Gentile nations in the Hebrew Bible therefore was not that they enter into the exclusive covenant relationship side-by-side with Israel, but that they observe the "proto"-law of loving ones neighbor as oneself. That is, according to the verse (immediately above) and Jonah 1:2, the specific "wickedness" of Ninevah was related to violence, which included animals with people. The Hebrew word here is חָמָס, which is the same word to refer to the immoral behavior of the antediluvian civilization at the time of Noah (Gen 6:11 and Gen 6:13), which was a time that also pre-existed the Law of Moses. The Hebrew word means violence or cruelty, which is violation of love for ones neighbor as oneself.
So in the case of Ninevah and Jethro, there is no reference to laws and statutes related to right relationship with YHWH, but instead to "God." These laws and statutes related then to the "proto"-law of loving ones neighbor as oneself. This law was not written, but oral. In the case of Noah, who had direct covenant relationship with YHWH, special revelation (before the giving of the Law at Sinai) also provided guidance on what animals were "clean" for the purposes of burnt sacrifice to the Lord (YHWH).