Hmm, as someone who has gone through a legal responsibility class, and has worked in a large law firm before, I will give my impressions too of the situation, this may help explain why the senior partner reacted positively. This is honestly a scenario straight out of an example question from a legal responsibility class, that illustrates some of the...difficulties of the current firm structure.
Some law firms, particularly the large ones, which are usually in or very near large cities, have something called an "up or out" system. Essentially, only a small number of associates will make it to partner. Many of the associates could be quite capable, and can do the required work necessary of a partner at that firm, but, there can only be so many partners made each year (this essentially is because the partners need to protect their share of the equity). As such, any major non-quality of work reason for denying partnership is considered important in making the decision. Something like an associate considering leaving the firm early is the most important one, it is extremely common, because the associates essentially have to look for other positions because of how precarious the "up or out" system is, but at the same time, any associate that made it up to the level of being considered for partnership, would want to know how loyal they are to the firm. This is why the partner would want to know.
Why the associate would do it, should be obvious, there is nothing complicated about it, and knowing about the "up or out" system doesn't really change the reasoning, it just provides a much greater incentive to do it. He did it so he can improve his chances at the firm, at the expense of one of his fellow associates, who he most likely constantly shares information and advice with on a very regular basis. Outing someone just for the sake of personal gain, rather than some ethical reason, is unethical. It appears that is the case here, because the son certainly did not seem to have any reason for it, other than to impress the senior partner. Perhaps there are other facts missing though.
Not going to comment on the legal questions, but I think you should give the SIL a second chance to prove his integrity, unless there has been other evidence in the past of a serious lack of integrity.
