Taylor Swift and Kendall Jenner seem to have an unsettled "feud" between them, despite having common friends.

This "mounting" conflict "for a long time" now comes to the point that Swift or "Tay-Tay" to her fans, is suggesting Kendall, who did not appear in her "Bad Blood" video despite being invited, needs a "girl code" lesson for dating friends' boyfriends or even exes like Harry Styles and Justin Bieber, a source revealed to OK! Magazine.

Apparently, the country singer was not in favor or Jenner's relationship with her former boyfriend, Styles, whom the "Blank Space" singer dated from 2012 to 2013. Jenner on the other hand, was rumored to have dated Styles from late 2013 to early 2014.

"Harry's always been the biggest issue between them. Taylor thinks it was completely disrespectful for Kendall to be hooking up with Harry after he broke Taylor's heart," shared the source to the magazine.

To make matters worse, Jenner allegedly got involved with Bieber, who was Selena Gomez's on and off boyfriend. Jenner and Gomez were friends, too, and Gomez is really close with "Tay- Tay."

For Swift, Jenner "came between Justin and Selena" and she just "can't be trusted," the source told the magazine.

Another insider shared how Gomez allegedly caught the two in April.

"After the Coachella festival last April, when Selena and Justin were still a couple, Selena showed up at his house and walked in on him and Kendall in bed together, fooling around. Selena was heartbroken and literally freaked out," an insider told Life&Style magazine, with the quote published on Hollywoodlife.com  

However, the source claimed that Jenner was not sorry to her former friend over the Bieber incident.

"Kendall thinks that she's done nothing wrong and has only tried to be a good friend to Selena...She insists that Justin is the one who came onto her in the first place, not the other way around, which makes her innocent and not a bad friend," the insider told HollywoodLife.com.

The rumored issue between Jenner and Swift is something that their common friends would have to bear. The two can actually have a choice to either abandon their friendship or be civil with one another like how they tried in December while decorating Hadid's house for the Holidays.