hybrid sytem stop immediately message low inverter coolant

you can probably drive slow, but don't add water. get some inverter coolant at the dealer, and start checking for leaks

 

I would make a dealership appointment and only drive it, gently, to get to that appointment (whenever that is, say next Tuesday, what with the long weekend).

 

Distilled water temporarily probably won't hurt anything, but it will be a trick getting the coolant to the right dilution again afterward, because the Toyota SLLC is sold already premixed at the correct 50% dilution.

In the ol' days coolant would be sold straight, and you could add about as much of it as the distilled water you put in, and end up back at 50%.

 

Hmmm......let's see.....where he lives, the temperature goes below freezing.......how many days a year ??
I think ZERO.
Adding water should be fine.

 

Would 1/2" below low line in hybrid coolant reservoir trip a warning? I kinda doubt it, suspect the invert coolant pump failed, but who knows.

 

That could be where it's leaking.

 

Water should be ok temporarily, as long as the right concentration of SLLC (which has a corrosion-protection purpose, not just an anti-freeze/anti-boil purpose) can be restored soon.