We asked a therapist what she would tell Maddy, Nate, and Cassie about their toxic love triangle in 'Euphoria'

• On "Euphoria," Cassie enters a secret relationship with Nate, her best friend Maddy's ex.
• When Maddy learns of their secret romance and sex life, she's furious. The feud has caused internet debate.

On HBO's hit drama "Euphoria," a longtime friendship between characters Maddy Perez and Cassie Howard imploded when Maddy learned that Cassie had been having sex with Nate, her ex.

The tense television moment became internet fodder when people jokingly mused, "Maddy or Cassie?" and likened the ex-best friends' feud to the Super Bowl.

According to therapist Kristie Overstreet, it's common for friendships to end after a situation like the one between Cassie and Maddy because it signals a lack of trust.

Even though sleeping with a best friend's ex isn't cheating, it can feel like a similar betrayal, Terri Orbuch, distinguished professor at Oakland University in Michigan and author of "5 Simple Steps to Take Your Marriage From Good to Great," told Insider.

Most people assume their close friendships come with mutual support and respect, which could include talking about romantic relationships or being there for each other after a breakup. When someone you believe is part of your support system becomes intimate with a person you're grieving, it can feel like your friend doesn't really support you, Orbuch said.

No matter how much time has passed since the breakup, or the terms it ended on, it's best to assume your friend wouldn't want you getting close with their ex, Overstreet said. But if you find yourself in a position like Cassie's, she says taking responsibility is key.

It's on Cassie to repair the friendship, a therapist said

If Cassie ever wants to repair her friendship with Maddy, she'll have to take initiative, Overstreet said.

Though Cassie didn't intend to harm Maddy when she decided to get together with Nate, her actions have repercussions and she should take responsibility, according to Overstreet.

"It's not frequently that the person is like, 'Oh, I'm just going to go after her man.' It's more of a person's individual struggle, that they're dealing with something," Overstreet said.

In "Euphoria," Cassie was reeling from her own breakup and dealing with self-confidence struggles. Her inner turmoil, plus being in close proximity with Nate, likely influenced her actions, Overstreet said.

Ultimately, it's up to Maddy to decide if she wants to rekindle her friendship with Cassie, she said. If that's the case, Cassie must apologize and gain Maddy's trust back, which takes time and consistency, according to Overstreet.

She added that, in an ideal world, Nate would apologize for his actions too. Still, Cassie's betrayal will likely hold more weight than Nate's because of the nature of their prospective relationships.

"A lot the time, the guy in the situation escapes because so much of the disruption happened around the two friends and there's not a lot of responsibility or accountability from his end," Overstreet said.

It's possible for a friendship to rebound after one friend sleeps with another's ex-lover, but it's a rare occurrence, said Overstreet.

Next time, ask yourself if it's worth losing a friend for romance

For that reason, Overstreet suggests you think twice before you get close to a friend's ex.

She said she can understand why Cassie, a teenager who is still growing mentally and emotionally, would make a potentially irreparable misstep in her friendship.

"We just aren't at that developmental place, and we haven't had those relationship experiences, so it's not far-fetched to find ourselves in these situations, especially with that internal conflict," Overstreet said.

But as adults, it's important to ask yourself, "Am I willing to lose this friendship?" if you're considering a move like Cassie's, and take accountability for your decision, said Overstreet.