Maybe you can mix those kits. Just because both are on the QVL does not mean they will play nice with each other. One set is cas 9 and one is cas 7 and that could very well cause problems.
Why exactly does she want more than 8GB of RAM anyway? Unless she does extensive Photo and video editing 8GB is more than enough.
If you want less headaches than trying to find a happy medium of what cas latency to run when mixing those kits it would be much better to add an identical kit. If you do try to run them together try cas 9-9-9-24 2T first.
She is an illustrator who works with InDesign, Photoshop, Illustrator and other Adobe programs. She is often running several at once and she just decided she needs Photoshop to run faster with a recently purchased a Wacom Cintiq 24HD. Do you think 8GB is enough? It might be safer just to chuck the two 2GB sticks. I did find these G.SKILL sticks
at my house. While they are not directly listed as compatible on the memory support list might they be safe to use alone?