My car leaks anti leak sprayed all over hood of car and everywhere dripped on floor?

Pretty sure it's anti freeze cause my stepdad is a mechanic and changed the thermostat and I guess that wasn't the problem but it leaked when I got to my friends house parked it I could see the vapor of it coming from my hood I opened it and it was sprayed all over the place I checked under and there was good 1/2 liter of it on the floor how bad could it be its a 2002 Honda civic coupe

You need to locate the source of the leak. Common places are usually hoses that are cracked or not secured tightly. Could also have a hole in your radiator, a bad cap on the radiator or overflow reservoir, bad seal on one of many parts, broken water housing, etc… In order to properly fix the problem you need to find the leak. With the engine cool (not ran for at least 4 hours) open your hood and look for anything constantly dripping. If nothing is leaking with it off start your car and watch the engine compartment as your car runs. Once it gets up to operating temp you should be able to see where it is spraying from, just be careful to not get sprayed as it will be hot. This should, if nothing else, allow you to pinpoint the area it is coming from. Shut the car off, wait for the spraying to die down some and start hunting for the source with a flashlight.

Was it doing this before the thermostat was replaced? If not that would be the first place to check, might just be your stepdad didn't use the gasket or it was defective.

Pressure test your system, you will find the leak right away.
Can be a pin hole in a hose, loose or cracked clamp.
Watching for it to leak might not produce any results, a pressure test will.

Locate the leak