Got into a fight with my best friend about an ex? What to do?

I got in a fight with my best friend (warning kind of a long story, like REALLY long)

So I have a best friend who I can tell anything to... She tells me anything. She once had a boyfriend and he was a cool person. We used to go to the movies together, us 3.
Now... They broke up. She still feels something for the guy. It was a harsh breakup. She doesn't know how he still feels about it. So She has heartache, hasn't found closure yet.

Me and him still talk on and off. No flirting or anything, we're just friends. We just talk every now and then just like we do with a lot of friends, to catch up on things.
And in the meantime I still see my best friend almost daily, we talk daily.

Anyway, one day I canceled our daily call because that ex wanted to talk to me and I was like sure why not. Because it's been a while that I've spoken to him and we got along well, I didn't see any harm in it.
The people who know me, know that I just went for the direct honest approach and told my best friend that I'm going to talk to her ex for a second, because it's been some time. My best friend felt betrayed, and she wanted some space. Now she's still having her space, I don't know how it's going to go.

My question. Should I have dropped a friend, because of a best friend? is my best friend being a bit unreasonable because?
+1 y
Details: Like 6 or more months prior me and my best friend had a long talk about her not being able to move on. But 1. She's in a new relationship, 2. She didn't bring up her ex after ALL THIS TIME.
So I thought she moved on by now.

I always had this mindset of... There are people who I get along with, but I haven't seen them in months or even years. If I got a chance to catch up, I would gladly do so and cancel plans I have with people who I'd see everyday.
+1 y
Of course, this person happened to be her ex, but me and that ex are still cool. I don't like to pick sides and drop a friend because of another friend (in this case, my best friend)

So please... Was I in the wrong? What should I do now about this, should I take my distance from the ex? She literally feels pushed aside.
+1 y
Thanks everyone for helping me out and giving me advice and their view on this.
She told me to choose between him and her.
After thinking for some time, I decided to drop the ex and cut off all contact to keep the frienship of me and my best friend. I did tell her though that I find it ridiculous and selfish on her part and that I will never agree with the reasoning, but that my bond with her is worth more than my friendship with the ex.
She accepted that and we managed to fix this.
Got into a fight with my best friend about an ex? What to do?
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