I’ll give a real-life practical example so we're not just reciting theory here. I knew a man who became Christian in his 20’s or 30’s, grew in faith, along with hope and love as he continued in his walk with God. He had the usual struggles and dry periods, backsliding at times but overall growing nearer and nearer to God over a period of years. Now, much later in life he was very surprised to find himself in love with a woman not his wife. All the thoughts and desires that come with that territory came flooding in and the human mind is quite capable of justifying adultery because it can just plain seem so right and natural. Love, itself, might even seem to compel it.
The value to not commit adultery does not hinge upon the 7th commandment. it's there implicitly from the marriage covenant, which is legally binding. When you set it as the law of your life, the product of which will be not to commit adultery (or at least that's the goal). This is a good product/goal, and I'm not saying these are not good things to keep, but they don't touch upon the deeper heart issues or values where our motivations come from. This root issue with your friend is not whether adultery (or having a second partner, etc...) is moral/immoral. That's the superficial symptom of a much deeper-seated issue. He may blindly keep the 7th commandment and not commit adultery, but this does not critically address the deeper moral compass that tells him to go somewhere else. This is the problem of the 10; they do not critically address the issues, but rather are focused on outward superficial morality that hopes to calibrate inwardly. Christ's approach is flipping this, where we address the inward first, and our outward actions are an outflowing.
I don't think the answer to your friend is holding the 10 commandments at a higher value and doing so would be just a band-aid solution (even if the product is not commiting adultery). What I'm sure we both can agree on is Christ is his answer and is a better moral compass over the 10. He is confused on if the HS is guiding him to immoral actions, and sure we can use the 10 to emphasize universal morals, but when you envoke the 10 he would be right to call out polygamous practices of the OT to consider the morality of more than one partners. You've opened the door to OT morals when you bring up the 10, your friend won't have the same value for your post-biblical rational separating the 10 like this. He will rightly see a tension that polygamous values are compatible with the 10 (as they were when they were presented). And you've introduced this by not critically approaching the issue and just slapping the 10 over it, saying it's the solution, so don't want the finger when he mixes in other OT values that are logically consistent with the 10.
the NT does not value adultery, and there are plenty of areas to draw from to show that this is not NC practice (polygamy is a bit of a different issue but we can keep it simplified). When we isolate the 10 and introduce it like a mic drop, then someone spiritually immature will see this as cause to celebrate any value they see in the OC in like manner that satisfies their thought process. Because you don't critically approach it, they, too, are not critically approaching adultery, and just using cut-and-paste examples to justify their actions (because that's exactly what you're doing with the 10). Christ teaches that we are to love our neighbour as ourselves. This man's wife is his neighbour (in a unique context) that he is intimately involved in a lifelong covenant relationship (marriage). introducing another partner violates this agreement (legally) and no doubt violates his wife desires (this adultery). Does he want his wife to introduce another man, too? If the answer is no, he has the answer for himself of what his action should be. Now the cause to not commit adultery is not because the 7th commandment told him so it's because he knows that if his wife did the same to him it would hurt him, thus the same action to his wife hurts his wife. This alone is an action of love towards each other over love of self (leading to adultery), and Christ tells us it's what he desires, so now it is a spiritual act of worship to love our wife in this way. The 7th commandment doesn't say that, and it's only a superficial goal (The real goal is acting in a way that loves your wife, not just with adultery but in all things) and the 7th doesn't teach us this.
This extends to the 4th too (which is the more critical issue here). The 4th does not address critical action. We are simply prescribed what to do on x days and what not to do on y days without critically identifying its moral value. the justification is to mirror the 7th day, but there is still no critical response just a string of whys that only ends with "because it says so" if we want it to impact us at a core level, then we need to critically deconstruct the 4th commandment (and with it the 7th day to address what it's core contructs and values really are, whether they are about physical value, or spiritual. When we unpack it's core construct and values we can address them as universal over their superficial counterparts that we see in the 10. Just like with adultery, it misses the point to just say do to steal, do not murder, do not commit adultery, etc..... I can figure that out without the 10 (and so can your friend). What but aligns my core heart action? because those thigns are just an outward focus, not an inward core drive. What is my core drive? you friend would be better to focus on that core heart value over parroting the 7th commandment and the 10 do not address this core. But Christ does this without smuggling in the 10 into the NC (or any other values surrounding the 10 like polygamy or salvery)