I think you are asking how to merge part of your friend’s tree into your tree.
You first need to create a Project or a GEDCOM extracted from your friend’s tree that contains the direct line you want to transfer.
In both cases, there is a technique for selecting Individuals from a tree Diagram and using that list to choose what to transfer.;
See FHUG Knowledge Base ‘Create an export GEDCOM based on a diagram’
In your case, you might only select the direct line ancestor boxes in the Diagram.
The resulting Name List can be applied to either the Split Tree Helper or Export > GEDCOM File method.
After creating the pruned Project/GEDCOM, use the File > Merge/Compare File command to import those records to your tree.
There is advice in the FHUG Knowledge Base on the Split Tree Helper, Export > GEDCOM File, and Merge/Compare File commands.