For some students it is translating what they are doing with what they are seeing quick enough.
So they look, see the leg is forward, but by the time their brain is processing what they are doing, they are on the way down in the post, so they think they are on the wrong diagonal.
What I would try as a test, is to say “Up, Down, Up, Down” with the horse’s outside leg, and see if by listening to you, she can tell is she is right or wrong. If she cannot, then I think it is an issue with how her brain is processing what she is doing vs what she is hearing/seeing.
If that is the case, then I would just tell her “up,down,up.down” for a while until she can match her motions with the words. Then have her look at the horse’s leg, and try to figure out when she should be saying (not doing) up, vs down without worrying about the diagonals. Next have her match what she is saying with what she is doing. Finally see if she can go right from looking at the shoulder with determining her diagonal.
But…I would only work on diagonals perhaps every second lesson and for a short period. Do not make them into something more important than they are; sometimes people get too stressed about something to be able to learn it!