Use the Python subprocess module for this. I am assuming that you are using a version that is 3.5 or newer. In this case you can use the run function.

import subprocess

result = subprocess.run(
    ['python', 'script1.py', '--input1 input1', '--input2 input2'],
    capture_output=True)

# Get the output as a string
output = result.stdout.decode('utf-8')

Some notes:

1. This example ignores the return code. The code should check result.returncode and take correct actions based on that.
2. If the output is not needed, the capture_output=True and the last line can be dropped.

The documentation for the subprocess module can be found here.

A alternative (better) solution

A better solution (IMHO) would be to change the called script to be a module with a single function that you call. Then python code in script1.py could probably be simplified quite a bit. The resulting code in your script would then be something similar to:

from script1 import my_function

my_function(input1, input2)

It could be that the script from the other dev already has a function you can call directly.