Friendships are something that has degrees.
For example, people here can look at their previous jobs, or school - while you were friendly and goofed around with some of the coworkers or school mates. But then it not necessarely converted into friendships once you left. Its not because people started hating you and you started hating them because you left, but because that is not enough to forge a friendship that will remain if you stop spending time together.
Imho, Zagi and Med relationship is somewhat like that. They were sharing same rink and spending lot of time together, and had fun, but once that ended, there were not enough common interests to remain closely in touch and so on.
Diana's case is different here. First, its much more personal for her, since Eteri was obviously hurt by Medvegate and Eteri is Diana's mom.
Just a personal example here: my father is a university professor. When I was finishing school, my dad got this undergraduate student, lets say John. Then, John went through master degree and PhD with my dad, and at that point John was basically part of family. He stayed with us for a couple of months when he had $$$ issues. He would come to my parents house to work on research with my dad and so on. Then, at some point, John gave up academics and decided to move on and work in a bank. Now see, my dad is a very strict person, and he could get rather nasty - as he indeed got when I chose to not pursue academic career, and we didnt speak for over a year - and I totally understand John's decision of not say anything to my dad, and simply avoid and ignore him. But my dad was devastated with that (devastated as in spending over a year depressed). Now back to my point of view as daughter: both my father (for being strict and not tolerating weaknesses) and John were in fault. But my dad is my dad, and John hurt him deeply. John may be a nice guy, but i have this resentment about him. I dont want to be his friend, and I dont want to interact with the guy who hurt my dad like that, period.
THen, there is a second factor: Diana was heavily bullied by Medvedeva's fans (who demanded her to support Zhenya against Eteri), and it was quite unpleasant, so I wouldnt be surprised if she chose to avoid as much as possible simply because of that.