What was his intention?

Anonymous
Trying to get a better understanding of the situation:

My now ex and I have a mutual-ish ‘friend’ that we see at parties here and there. Anyways, a few months back my ex and I separated for about a month. In that time frame I posted a little bit more of a ‘risky’ photo on social media just feeling good about myself. Said mutual friend saw that photo and liked it. No big deal, didn’t think anything of it.

Fast forward to a couple weeks ago, my ex and I were back together and saw friend at a party. This friend mentioned, “why don’t you post pictures in your underwear anymore.” I immediately shut it down and laughed it off. (My ex didn’t know I posted the picture because we weren’t together but I took the photo down when we did work things out)

Later in the night my now ex approached me and said friend said “I think your lady hates me now because I said something about her not posting pictures in her underwear anymore.”

This caused a big argument between ex and I where I tried to explain and tell him it was one time, the photo is now deleted, and we weren’t together so I didn’t really do anything wrong.

But now I’m wondering why friend went out of his way to tell my ex after I told friend that he didn’t know about it. Was it on purpose to get us into a fight? Was it just friend being a “bro”? Was it something else? I’m just annoyed why he mentioned it to my ex after I told him he didn’t know about it and not to mention it.
What was his intention?
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