Over Heating Sign This is 2010 Prius with 120K

My friend just bought this Prius and what he realized that one of the hoses from coolant's reservoir from the front were cracked and reservoir did not have any coolant in it, so I bought the new hoses and replace the hoses for him, I also put the coolant up to full mark on the coolant reservoir tank.

When he was driving around after 15 miles, the red temperature light came up, I open the hood and I found coolant reservoir was full, also front hose were hot, I didn't see anything leak, or any white smoke.

Also, the check engine light came up I ran the OBD2 and I got to error code saying that engine coolant temperature sensor is not good

I looked online, and I got an instruction showing how to change that sensor also it was saying that empty engine coolant before changing the sensor

But when I took the sensor off, it did not leak any fluid from the hose, so I didn't have to empty the
engine coolant before changing the sensor

So I took the old sensor out put the new sensor in, and then I was driving around, and that red temperature light came up again right now the check engine light isn't on, so I don't know why the car is still overheating

I called the mechanic, and he said it could be so many things he said he has to see the car, but he said he's busy, so he would not able to look into it probably another 2-3 weeks

Previous owner was rough on the car, does not look like he took care of the car or maintain the car anyway

My mechanic also saying if you already drove the car 15 mi or 20 miles with a red temperature light warning on then probably engine is already done I don't know how true it is or not what is the further troubleshooting step should I take to see what causing it??? thanks