I'm going with master rayray on this one. All depending on ratio of amount dumped to total tank volume. It was only a gallon and given his tank volume and the current weather, it may have been the amount of evaporation and top off for the day anyway. Unless he's also dosing other stuff like two part or was mixing the kalkwasser with vinegar, then pH reading showing a no major spike then I think he'll be ok. Of course, also checking alk, salinity, pH and water change etc. wouldn't hurt.
If it was a major disaster, then we'd be reading a "tank crash" thread right now. hahaha! sorry man...shouldn't joke like that. Good luck!