I'll put in my .02. My in-laws disliked me intensely because I set high standards for their boy and expected him to function as an adult. They withdrew from our lives for the most part, never knowing our children, moved away to the south of the US. Never visited. So when H started going into MLC with all his disquiet and unhappiness, spouting (I'm just guessing here) about how I made him unhappy, I can imagine that his sister and his parents seized on that and encouraged him, "to pursue his own happiness". As H went ahead and abandoned me for his OW, shortly thereafter to be dumped by OW. Their little boy was left crying about how he "was all alone and didn't belong anywhere". It made his MLC exceptionally traumatic. Too bad, you get what you ask for-
Well, that is a situation that he created for himself but with their full support. The twist is this: now it would be uncomfortable for him now to admit that what his family and his friends (his only support system he has left, who listened to how unhappy he was and encouraged him to go after his happiness with OW), how they were all wrong now, for him to make an About-Face and come back after we have divorced. Surely they have no idea what the ultimate result was to him (i.e. huge unhappiness) or the emotional turmoil that he went through because he followed their advice (keeping in mind he is juvenile in his decision-making). I doubt he will ever have that much courage...
Created a bit of a hole for himself, I'd say. Overall I think that other people can influence the MLCer if he is looking for one of two things: 1) Support for his plan (and it aligns with the in-law's self-interest), or 2) he is a consummate co-dependent & conflict avoider such that once he throws the wife overboard, he must attach to something so it becomes his family of origin (who would never abandon him whatever he did). In those instances, I think that friends and in-laws are the enemy of an LBS if they are the type who preach "pursue your own happiness at all costs". My XH learned his avoidant behavior from his parents so they are not going to be the type to encourage him to face the pain, are they? Easier to coach him to run away from it all and vault off to OW, the supreme happiness.