are you allowed to be friends with a friend's ex?

Are you allowed to be friends with a friend's ex?

I had a situation happen some time ago and I wanted to talk about it. I am going to change names in order to keep these peoples' lives private & "try" not to piss anyone off. 
***This post's point is not about the friendship- it's about the the things leading up to this***

So I had this friend- lets call her Tina. Now, Tina and I hadn't been friends long. We met back in October of 2017. She and I hit it off fairly quick, but we weren't what you would call "best friends". We hung out most weekends for about a month and a half and got super close, but the friendship didn't have any real substance. We just went out to the bar and drank and went on double dates. I mean don't get me wrong, this girl is cool as shit! But she has a best friend already and I was cool with knowing that we were never going to be super close. 

Tina had a boyfriend- lets call him Conner. I had never met Conner until Tina. We didn't have any mutual friends or anything like that. I only knew him through Tina. So like I said earlier- Tina and I would double date often. Well my husband and Conner hit it off. They became friends pretty fast and it was nice to see my husband make a new friend. And I thought Conner was pretty cool too. 

Fast forward a few months later and Tina and Conner broke up. Now I wont go into detail of what happened, but we all know the saying; "There is 3 sides to every story- his, hers, and the truth". I just stayed out of it because at that point I was friends with both of them and I really didn't want to be mean to either  of them. I stayed friends with Tina for maybe another month...

One night Conner ended up hanging out with us. Tina and myself were drunk this night and got into a disagreement about a pretty touchy subject (to me). I didn't think anything of it the next day, because we were both drunk and it was small. If anyone should have been upset however it should have been me, because like I said- this was a crazy touchy subject that she brought up *in MY house I might add* & I tried to end the conversation plenty of times but she  kept going. I am not saying however that she isn't entitled to her feelings- because she is!

Next few weeks go by and I don't really hear from her- I text her a few times but it's like pulling teeth to get her to respond. Then one day I see her (won't say how- because I don't want anyone knowing who this is) and she made a pretty snarky comment that I'm sure she didn't think I heard... but hey, I did.
I get that fights can be awkward after the fact- but I didn't know that there was a problem- she wouldn't communicate. I am 23 years old, married, and living a fricken adult life; I don't have time for pettiness on that level. If she had just been honest and was like, "hey you pissed me off", I would have been more than happy to talk about it and work it out. I can't fix what I don't know! But unfortunately that's not what happened.

Again, this girl is awesome! She means well, she is kind, and thoughtful. Just an all around good person- it just didn't work out for us to be close any more. I think people are in our lives for a reason. I think we did something for one another & once that was fulfilled, life changed the game. 

So back to the ex. He and my husband were friends & honestly Stephen missed hanging out with this guy. So, is it so wrong to want to be friends with this ex of an ex friend? I don't want to do it out of malice; it's more so because we all got along so well. I don't know. ​