I think the whole thing is bogus and you need to find a competent mechanic and get a second opinion. The strut mount is two pieces of metal with a rubber block in between them and a bearing in the center. It holds the top of the strut in position for camber and caster while the bearing allows the strut to turn when the wheel is turned. It is all contained inside the strut tower and therefore is not visible for an inspection.
The chain store mechanic would have had to remove your front struts in order to see if it was cracked. I have had the rubber block on these crack, but as long as there is weight on the wheels, it would be hard to tell that the rubber is cracked. Even on a lift, the top nut on the strut rod keeps everything from falling apart.
If the strut tower is cracked, I'm not sure there is any really safe way to fix it, but the vehicle would likely have some accident history for this to happen.