1. Mako is everybody's friend. But, Tomoko was nobody's friend. Keyword being was.
2. Tomoko is a sleazy idiot that Yuri had to bail out, multiple times. Other people don't know that Tomoko is like this.
3. Yuri wants a bubble. She wants her three friends to always be with her. She can handle Mako and Delinquent going off with other friends. But, she can't handle them growing beyond the bubble.
So, It's hard for Yuri to accept that Tomoko is growing and she seems to be incapable. When Tomoko looked like the one who would never grow beyond her old self.