Look to see how busy the PC is. Run Task Manager and/or Resource Manager. I've seen PCs with a swamped CPU get laggy in a lot of different things including responding to key strokes. Also, running low or out of memory can make a machine run like molasses.
Is it all the time, or just at certain times? Maybe some background cleanup, virus check, or defrag is starting and eating up resources.
When they keyboard is laggy, does the mouse pointer also seem sluggish? That usually points to CPU or memory starvation.
What app is she using? Typing into Notepad requires almost no CPU, but typing into something like Word with heavy formatting being applied can eat a lot more of the machine.
What are the specs of the machine? CPU speed, disk speed, and physical memory can all affect how responsive it is. In fact I've seen laptops almost double their performance just by adding more memory.