We actually started out in the intro board, then moved to the blog board. Neither is as well-suited for giving/receiving advice as the Poly Relationships Corner
, it's likely to attract more people's notice (and invite more comments). If we start a new thread there, I suggest providing a link to this thread for any who desire a thorough background.
I think it was inevitable that J's relationship with A would become something you'd wonder and ask A about, as well as something A might want to start talking about with you. You can still be respectful of their space and offer help at the same time. Let A know that while you consider this a matter between her and J, you're still willing to offer your thoughts if she's interested, and, do consider directing her towards the discussion here.
For the moment, I don't detect a big problem but I suppose it wouldn't hurt to ask A about it if it's troubling you.