Personally, I wouldn't encourage her to convert inches to centimetres.
We are metric here, but every quilter I know uses Imperial. We do have to buy our fabric and ribbon etc by the metre however, so I usually buy 0.9 if I want a yard. Virtually everything quilt-related will be given in inches and parts thereof.
However, if she insists, the easiest way is to have a two-sided tape measure and flip it over to see the sizes. Also, one of my Olfa mats has a metric side and I know that Omigrid do a metric ruler.