She could trust elves, and Elrond in particular, but not dwarves. Or she could feel that the dwarves are Elrond's guests and that their visit is some kind of formal occation which she and her son, being lodgers in Rivendell, perhaps should stay away from. Well, maybe that doesn't sense. But the fact is that we have a situation with Gilraen and her son in Rivendell, Thorin and Company arriving and staying a fortnight, and no evidence of them meeting each other. So, we have to make something up. I think the easiest way to do this is by making Gilraen somewhat distrustful of dwarves.