She suspects something, but so far she's been getting stonewalled. As I mentioned before, the guildmaster probably wants to make sure Yuki had nothing to do with Dion's betrayal before sharing any info about him. Which is smart, really, when you look at it from the guildmaster's perspective.
She's trying to protect Dion. She only has the story from Dion's perspective. Because of that, she doesn't know whether or not Yuki was in on the betrayal. She feels it's safer this way because if Yuki was in on it and the Guildmaster told her where to find Dion, then Yuki might try to kill Dion to tie up loose ends.
Keep in mind, I'm not saying that is the case. Just that the guildmaster is trying to play it safe because she feels she doesn't have all the details.
Really, I wish people would stop acting like some of these characters are fucking
Gwenpool of all people... (That is, able to see the story from the reader's perspective and thus know exactly what is going on at all times...)