I don’t know if this can help, but did you try use review functionality? On a PR in the “file changed” tab you and your collaborator can comment the code and give suggestion.
On Github guides you can read this words:
Discuss and review your code
Once a Pull Request has been opened, the person or team reviewing your changes may have questions or comments. Perhaps the coding style doesn’t match project guidelines, the change is missing unit tests, or maybe everything looks great and props are in order. Pull Requests are designed to encourage and capture this type of conversation.
You can also continue to push to your branch in light of discussion and feedback about your commits. If someone comments that you forgot to do something or if there is a bug in the code, you can fix it in your branch and push up the change. GitHub will show your new commits and any additional feedback you may receive in the unified Pull Request view.
Pull Request comments are written in Markdown, so you can embed images and emoji, use pre-formatted text blocks, and other lightweight formatting.