Overheating can cause a number of issues, but cracked block is generally not one of them. It appears the owner wants to put this issue on the shop that winterized it. Do your research, have all the service records extending several years back. It will be difficult to determine when and how the block got cracked. Most importantly, be nice with the shop. Not to say an issue like this won't end in a court battle, maybe they will work with you to correct the problem. If you walk in kicking and screaming, your not going to get far.