Sorry, I didn't mean you should stop "spying" altogether. I just meant you'll drive yourself crazy if that's all you do.
A few thoughts:
It may be time to draw a stronger line in the sand. Tell her you know she says it's just platonic with B but even so, you're feelings as her husband should be at the top of the list of priorities. You could tell her that the frequency and closeness of interaction with B, especially outside the office, is more than even the most confident husband should be expected to endure. Explain to her that it's disrespectful, especially now that you've voiced your concerns respectfully multiple times. It's totally understandable for you to ask her to limit her time with him outside the office.
Also, you can say what though you feel your feelings are warranted and justified, that you're excited and curious to learn what you can do to be a better husband and meet her needs maybe in ways you haven't in the past.
Remember ask questions about her job and focus on her experience. You don't have to be an expert in her field to care about how she feels about her job and share in her successes and failures alike.
Lastly, check out the book "The Married Man Sex Life Primer". (I have absolutely no affiliation with the author or publisher). Yes it says it's about sex but it's more about exuding confidence and changing the confidence dynamic in your relationship.
Good luck! Just make sure you start getting to the heart of the matter. Stay consistent, kind and respectable and don't lose your motivation when she tells you it's nothing. The conversation isn't over until she agrees to stop spending so much time with him or she confesses what may be wrong in your marriage from her point of view (even if you think what she says it's unfair). This line about "we get along great" seems to be too convenient. Maybe there is a misunderstanding in the marriage that could be cleared up and improve communication.