Rule #1 of The Girl Code: Never go after your friend's ex.

You never flirt with him, try to sleep with him and you never talk to him behind her back.

It doesn’t matter if it was totally official or a few months of intense fling. If she’s ever cried over him, told friends she had strong feelings for him or gushed about him all the time, he’s “ex” material.

This rule is so well known, no girl should ever have to be reminded of it. Though many are aware of it, some girls choose to break it anyway, and it’s an unforgivable crime.

When I was about 18, I became very close with someone who I grew to have very strong feelings for. When I was 18, I also met a girl who became my best friend through my first few years of college. I thought both of these relationships would last longer than they did, and I put a lot of trust into both people.

My best friend knew all about this boy I liked.

She was in on every detail and update. She listened to me talk about him, helped me flirt with him, and eventually when I had enough of him not willing to communicate and reciprocate my feelings, she sat with me through the heartbreak -- which for a college freshman, you basically think it’s the worst thing to ever happen to you.

She witnessed me go from head over heels to heartbreak in a matter of days and even encouraged me to ask a guy from class out just to boost my mood. She supported me.

I don’t quite know what happened to start everything, but all I know is a year later this ex was back, but not necessarily in my life.

It started with a joke swipe on Tinder to see if he knew that she was my best friend. We’d plot messages and be difficult with him, but somehow I became less of a part in the plan.

She’d talk about messaging him and playing stupid still, but I never saw messages. Then they were Snapchat friends, but the best friends feature was long gone and the only way to know a person’s username was telling them. Then it turned into liking each other’s Insta pics, including the ones with both me and my friend.

He knew who she was to me, but he didn’t seem to care, and I didn’t care about his part in all of this.

How did it get so far to where she was getting asked to his formal in another city? Why did I notice there were parts to the story I couldn’t see, but not care? Why did someone, who said they supported and loved me, break the number one rule of friendship?

I never gave my blessing for her to get buddy-buddy with him. I knew he was talking to her as more than a friend, and I knew she was talking the same way.

Friendships are about trust.

Period.

I put all of my trust into someone who used it against me to talk to a boy -- my ex of all people.

In the same way, you want to stop putting trust into boys after a break-up, I stopped putting so much trust into girl friends.

Thinking about the way I was given the runaround still upsets me. I think about how long I was blind and get frustrated with myself.

I remember when my best friend asked me if I’d be OK with her going to my ex’s formal, and I remember how it felt like I got punched in the chest and I tried not to show how upset that question made me.

At first, I thought I was more upset -- bitter even -- that I never got an invite like that, but he wanted to invite who he knew was my best friend. Thinking back, I can still feel that punch to the chest, and I know it’s because she put him before me.

She knew the code. She knew it was wrong to talk to him, and she knew it would be wrong to accept his invite, but she took the risk and asked anyway.

So, old friend, in case you’ve forgotten the number one rule of Girl Code in our time apart: stay away from your friend’s exes, for it’s an unforgivable offense, and I, for one, will never forgive you for breaking it.