Well, have you asked her HOW she's doing the problem in her head?
Like-- is she visualizing beads? Is the problem that she can't "hold" the image? Maybe it would help to arrange things in rather fixed "patterns" so that she can recognize "shapes/patterns" (like the number of symbols on playing cards, for example-- you don't have to "count" an 8 to know how many hearts are there, right?) and speed things up that way.
I tend to work basic math quite visually, myself. I'm terrible at rote memorization, so fluency was a VERY slow go for me personally.
Is this difficulty in the context of two-digit subtraction? Is she also having trouble with addition, or ONLY with subtraction?
I tended toward the latter-- for some reason, that is more complex to do mentally if you're working problems visually. I think it requires an extra "holding/manipulation" step over addition.
