My personal opinion on it is that you need to work out how to deal with your emotions concerning the jealousy about the secondary. Like everyone else is saying, is he okay as a person for her to go to. Do you think that he is going to use that information against you. Do you think he's the type of person to tell her things like "Well if its that bad then leave the relationship" and not things like "how have you considered working this out?" Will he use this information against you, or are you just FEARFUL that he will use it against you. Has he done anything to make you fearful of this information.
If not, and he has proven to be a good supporting role, giving her advice, good advice as its needed, you have to remember that when you feel jealous and learn to look past that.
In the meantime, while you're trying to work through that issue, let your partner know, "Hey I'm trying to work through these feelings and not restrict you, for your secondary is not a bad person and I know this. So I ask that you work with me on this and let me know what you're going to discuss with him." or something along those lines. But actually work through those feelings, don't just keep constantly giving that excuse.
Good luck okay.