Do you know the context in which these pictures are used?
Are they part of a proposal to clients? If so, what sort of sized project? There's a difference between the costs she'll absorb on a £10,000 project to what she'll absorb on a £10m project.
Are these images part of cost that she passes on to her client, or is it her overhead?
Do you know the importance of the images to the proposal? Is it a case of her using them if she can get a good rate, but would just omit them if they're too expensive, or would she have to get them done elsewhere?
You need to compare the cost of the prepared images from other people, not the hourly rate. If someone else can do the same work, but is twice as quick, you can't charge a high rate because you're slow.
These are the types of questions that will tell what what the market can bear.