I get that they eventually ended on good terms (the initial break up was very bad, but they eventually came around) and are now friends, but is it right to be a little concerned that he still has a decent amount of contact with her (i. e. he replies to her stories, she comments on his picture, etc.)?
Most Helpful Guys
If your in contact with an ex at all, it should be limited to at most probably once every 3 months-ish. At least, that is my opinion. That is keeping plenty of distance where they aren't involved in your life to the point where they might disrupt it (whether emotionally or otherwise) such that you are confident you can keep them at a 'Grinch's Length' but allows for things to not end up in a way where you would feel like you just completely discarded someone who you spent a good chunk of your life with, which could feel morally wrong as well.
ANY contact with an ex is too much. You should both go your separate ways and move forward with your lives. Contact with an ex only presents the opportunity for your new companion to have questions about your availability, allegiance, and desire for them.
Most Helpful Girls
If the contact between them is only online then I wouldn't be concerned. If they're hanging out in person I think that would be an uncomfortable situation. I wouldn't mention anything to him if it's an online friendship, even if they flirt a little, as long as he's being honest with you. You can watch and see if anything seems to develop w them. This shows confidence in yourself. If anything does happen later between them then you reevaluate the situation at that point
It really depends on the content. But you have to remember that they probably dated because they were friends.
It depends on how much you think is too much. If you think 0 is the right amount but he's still talking to her then you'll be unhappy. If it doesn't bother you much, then it won't matter to you. It all depends on your threshold of tolerance for it.