She doesn't wreck the married couple. It was the married colleague all along. He had his midlife - crisis and would have fucked another young girl when it wasn't Mari. ANDR the wife was a foreigner to her, mari was his Best friend, so his betraying behaviour has more weight.
No, she did have a hand in "wrecking" the family. She knew it all along that he had a family, and still did the bitchy/asshole thing. If the midlife-crisis dude fucked some other girl , it would be half her fault as well (if she was aware). She had a hand in "wrecking" the family , because she Is The One Who Fucked The Midlife-Crisis Dude.
Also , not everyone agrees that a a friend's action carries more weight (I sure don't), that's just Your Subjective Opinion. Please don't present it as an Objective reason.
She apparently has a thing for married men, going on Junyoung's earlier narrative, which is why he assumes that all of her "boyfriend oppa's" are married. Which led to the big misunderstanding with Shihoon as Mari calls her brother by the same term she does with her boyfriends. Mari has no shame, her behaviour in front of the now ex-boyfriend oppa's wife showed that. She doesn't think she has any responsibility for her actions, and blames everyone else for her own poor behaviour.
Let’s leave it in Mari’s a bitch
Mari DARES to call my baby a traitor?! SHE WRECKED A MARRIED COUPLE, AND MY BABY DID HIS BEST EVEN IF HE STARTED TO FALL FOR THIS GUY?! WHAT THE HECK MARI