I do think it inappropriate for young Jolyon to talk to Bosinney, notwithstanding his long separation from June and the circumstances behind it. Young Jolyon's experience is more similar to Irene's, and Bosinney is in the same position as young Jolyon's second wife (Helene). I found the following interesting, because it made me wonder what young Jolyon thinks of his second wife, or what she may think of herself:

He [young Jolyon] had not fled, nor would he fly if it were all to come over again. Yet he had gone further than Bosinney, had broken up his own unhappy home, not someone else's.

I think the wrong parties are talking here! With these two couples, young Jolyon and Helene (his second wife); and Irene and Bosinney, the similar circumstanced parties should be aligned as follows:

Young Jolyon = Irene (these two are the married partners who want(ed) to break free of their spouses, they both share(d) desperately unhappy marriages)

Bosinney = Helene (these two are similarly circumstanced in that they are the interlopers, or individuals who threaten pre-existing marriages)

So, if you can follow my poorly-worded and convoluted logic, young Jolyon should be addressing his concerns to his counterpart in this matter, which would be Irene. It is their experiences of unhappiness which are similar. Ditto Helene and Bosinney. Of course, it wouldn't be seemly for him to talk to Irene, just as it probably wouldn't be acceptable for young Jolyon's wife (Helene) to speak of her experience to Bosinney.