There are many cornerstones of friendship, but trust and loyalty are two of the most important ones.

But as the woman in this story found out, trust and loyalty can only go so far – and sometimes a friend’s behavior crosses one too many boundaries.

Read on to find out how her best friend’s behavior threw away fifteen years of friendship, and how it all ended up blamed on her.

AITA for telling my best friend’s girlfriend that his “work trip” was actually a beach getaway with another woman?

I am a 30-year-old woman with a male best friend, Mark (also 30).

We’ve been close since high school.

After college I moved back to our hometown and we both still live here.

Mark feels like family. we’ve helped each other through deaths, breakups, health scares, everything. I love him like a brother.

And over time, their friendship continued to grow.

About three years ago Mark started dating Izzy (28, female).

He was nervous for me to meet her, like my opinion mattered.

I loved her immediately! She’s funny, smart, magnetic.

We quickly became friends ourselves and my wife and I often go on double dates with them.

But then, things started to change.

A few months ago Mark started a new job and began posting many photos with a coworker, Jill.

Most were clearly at work, so I assumed she was just a teammate.

I didn’t think much of it until one night during one of our usual double dates, when things felt off.

When Mark went to the bathroom, Izzy told my wife and me they’d just argued because Jill had been flirting with Mark.

So this woman decided to probe a little deeper.

The next day at the gym with Mark (we work out together sometimes) I asked about her.

He said Jill is around our age, recently divorced with three kids, and helped him settle into the job. And they’d gotten close.

He admitted she flirts but claimed it’s harmless. Izzy saw a text from Jill that upset her, but he said it was nothing.

I told him posting photos with Jill was disrespectful and to set boundaries.

But the drama was only just beginning.

He didn’t.

Jill kept commenting on his posts and called him for “help” at her house (like fixing stuff, or chasing off a bat) while alone with her kids. Mark always went.

Our double dates slowed, and he started complaining Izzy works too much and doesn’t encourage him.

Two weeks ago or so Mark told everyone he had a work trip. He went, came back, and everything seemed fine.

Gradually though, things got more and more suspicous.

Then last week at the gym he wouldn’t stop talking about Jill. He said he wanted to host a BBQ at his and Izzy’s house so I could “finally meet Jill.”

He hyped her up the same way he’d acted years ago introducing me to Izzy. That set off alarms.

I pushed, and he finally admitted his “work trip” wasn’t for work at all. He’d gone on a weekend beach trip with Jill, her kids and some friends to celebrate her birthday. Izzy had no idea.

Mark said Jill made a move and they kissed, but swore it was just that and they agreed to stay “friends.” He still planned to invite her to a BBQ at the house he shares with Izzy without telling her.

Read on to find out how the woman reacted.

I was furious. We argued. He said it wasn’t my business. My wife suggested waiting to see if he’d confess.

This weekend Izzy texted me saying Mark was acting distant. I couldn’t take it and told her everything.

She never replied, but yesterday Mark blew up my phone saying Izzy left him, calling me a “snake” and a fake friend. He said after 15+ years of friendship I owed HIM loyalty, not Izzy.

I thought I did the right thing. Now I’m second-guessing.

AITA?

It’s all well and good waiting for Mark to do the right thing and tell Izzy, but it seems quite clear that he wasn’t doing so and Izzy was going through more and more turmoil as a result.

Loyalty to friends is important, but when those friends are hurting other people, sometimes other things are more important.

Let’s see what folks on Reddit had to say about this.

This person agreed that she’d done the right thing.

While others explained that friendship doesn’t mean blind loyalty.

Meanwhile, this Redditor pointed out exactly how much Mark sucks.

Mark is the only one who has done wrong here, and it seems that he’s completely unwilling to accept that.

He could have kindly stopped the woman’s advances months ago, but he chose not to, and now he’s in a mess.

And he only has himself to blame.

If you enjoyed that story, read this one about a mom who was forced to bring her three kids with her to apply for government benefits, but ended up getting the job of her dreams.