Usually....inspectors or auditors will have raised eyebrows around the use of non-food grade rock salt in that application even though it is indirect. Why? Because you can have a crack in the jacket which can lead to the salt leeching into the ice cream. Sure, it doesn't sound likely, but IT DOES happen....ask me how I know.
This is why when you use glycol coolant in heat exchangers you must have food grade glycol, and the same with steam for the same reason.
It isn't your problem necessarily, but something your customer will run into at some point. As far as you pouching it for them, so long as you follow Tim's guidance you should be good. It will be questioned by auditors and inspectors in your facility, but if you can show how you eliminate / minimize the risk then you should be ok. It is assuredly an oddity...