Just to clarify, the python script is run completely independently of Node-RED? It is not Node-RED that will be starting the script.
But when the script does run, you want its output to get to Node-RED in some form.
In which case you need to use some form of interprocess communication. A few options have already been mentioned; the python script could publish messages to a local MQTT broker (eg mosquitto) and have Node-RED subscribe to those messages. That would fully decouple the running of the script from Node-RED receiving its output. You could configure a Node-RED flow to listen on a TCP port for incoming connections - your python script could connect to that port and write its data.
There are lots of options - all depends which approach you feel comfortable with.
