How would you handle this group dymanic and tension?

I'm going to detail this with made up names of the people involved. Mark is in a group where he is close to a few people including Karen. Eventually, Gwen joins this group. She starts to really blend in well with the group and seems to be having a lot of fun with everyone. However, Karen has been giving Gwen the cold shoulder and would not respond to Gwen when she even tries to greet her. Mark was the one that got Gwen into the group. When Gwen is talking and laughing with others, Karen interrupts and completely distracted the other person away from Gwen. For a while, it seems like Mark has been showing a lot of favor towards Gwen, even following her around. But Gwen doesn't seem to notice at all. Another guy in the group who is Mark's best friend, is completely infatuated with Gwen and mainly talks to her when she is around. Eventually, this best friend, we'll call him Kenneth, starts to walm away from talking to Mark in several occasions to talk to Gwen when she passes by. Not long after Mark sees this, he completely does a 180 on Gwen. Instead he is now Karen's biggest supporters and seems to want nothing to do with Gwen anymore. As a result, Karen is a little more nicer towards Gwen, who seems oblivious to the dynamics.
How would you handle this group dymanic and tension?
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