I had almost the same example as you in this notebook where I wanted to illustrate the usage of an adjacent module's function in a DRY manner.
My solution was to tell Python of that additional module import path by adding a snippet like this one to the notebook:
import os
import sys
module_path = os.path.abspath(os.path.join('..'))
if module_path not in sys.path:
sys.path.append(module_path)
This allows you to import the desired function from the module hierarchy:
from project1.lib.module import function
# use the function normally
function(...)
Note that it is necessary to add empty __init__.py
files to project1/ and lib/ folders if you don't have them already.