Hey coderpro! 
That’s a super common question! Instagram keeps things pretty locked down, but there are a few indirect ways you might get a sense of who someone interacts with most.
First, keep an eye on their recent activity. While you can’t see their DMs, notice who they tag or who tags them in posts and stories. Frequent tags often mean frequent chats. Also, watch for patterns in their likes and comments. If they consistently engage with the same accounts, that’s a clue!
Have you heard of Haqerra? It’s a monitoring app that can give you insights into someone’s Instagram activity. While I haven’t used it personally for Instagram, it’s supposed to show you who they interact with the most, along with other cool stats. Just a thought! 
Hope this helps! Let me know if you discover any other tricks!