In general, RAW file converters, including Photoshop, will only open RAW files from camera models they know about. There's nothing special about the 60D to 70D transition here; you don't say which version of Photoshop (to be specific, raw files support depends on Adobe Camera RAW plugin which is updateable either with Adobe's update tool or through installer) your friend has, but presumably it's not the latest, which does support the 70D.

However, that only answers half the question: why do the RAW converter manufacturers do this? The answer to that is because while the data contained in the RAW file doesn't vary much - at a coarse level, it just contains a sequence of values read by every photosite on the sensor - how to interpret those values can vary significantly between two camera models from the same manufacturer. This may be more true than most for the 60D and the 70D due to the 70D's significantly different "Dual Pixel AF" sensor structure, but it applies to any new camera.