I don't like the idea of auto antifreeze, it could contaminate their 'potable' water if makeup water has to be added to the hydronic heat system. They'd ought to use nontoxic antifreeze used for just this type application, along with potable water lines and toilets etc.. You find it at most plumbing supply houses and camping equipment dealers (if not Lowe's etc.).
If they were to drain and blow out the water lines, they wouldn't have to worry about them freezing. And I'd suggest doing that.
Quality Water Associates