Best Friend is Kind of Controlling

So my best friend is a guy and he has a girlfriend, I met her a while back and at some point she followed me on Instagram (and I followed her back). We’ve exchanged phone numbers. However, my best friend limits what I am allowed to send to her. I am not allowed to send her anything where he looks like he is doing anything remotely reckless or is somewhere he doesn’t want her to know about. (We originally exchanged numbers so I could send her bad or funny pictures of him) He also didn’t want me to add her on Snapchat because he didn’t want her to every see him potentially on my story doing something that she might not approve of.

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I have no idea why, should I ask about it or leave it alone? Do you guys think it’s kind of controlling or is it just me? I also just think it’s weird in general that he doesn’t let her know about certain parts of his life considering they’ve been dating for like 2 years.