A quick fix is to put colored tape on the reins - something that stands out like white tape on black reins. There are also reins you can but that have colored "sections" - so if you tell her BOTH hands should be on the line between blue and red then she can easily see if they are located in the proper place on the reins, and of course if they are even.
He's a link to an example of what I'm talking about as far as colored reins go:
Also - you stated "She's never liked holding the reins too firmly coz she doesn't want to pull on the horses mouth" - teach her instead of opening her fingers to open (and close) her elbows. I like to use the example of holding their hands - does anyone enjoy holding "fish" hands - where the contact is so light it's like holding a slippery fish? Conversely someone who crunches your (hand) bones is also uncomfortable. So best to keep a steady "medium" contact, which she can do by opening and closing elbows for a nice steady contact.