Help understanding code to build circuit for plant reader
Pin13 is attached to the onboard LED.

So when you see a reference to it, it means that the program is turning the light ON/OFF. You do not need to do any wiring with it.

But the logic seems strange: it turns the LED off only when the analog reading is out of range (and it resets the stress).

It seems to imply that once the plant is stressed, it will stay stressed :-)

Btw, as side note: posting the code in ASCII form would allow people to quote/comment on it. The screenshot is not very good for that purpose.

What cannot be figured out from the code, is how to instrument your plant.
If your friend has a working setup, you could try to replicate it.
At least post a few pictures.