I have a repo on GitHub. Someone forked that repo and is doing some work in their fork. I want to fork their fork to help them before they do a pull request to get the repo
I would recommend making a new branch off the branch you are both collaborating on, do the pull request and resolve merge conflicts there, then, when all is well, merge your branch into the development branch you guys are working on.
The way this is normally handled, is he would fetch from you before he does his pull requests and he would resolve his own merge conflicts as he would probably know best and this frees you up from needing to do that. Another option, is to pull and fix conflicts yourself.
In a larger project, there would be one person who has this job, to solve all merge conflicts, In companies, this would be the person selected for doing deployment.
Good luck!
Source: Tech Talk: Linus Torvalds on git
Please watch the source, as it really gave me an understanding on collaboration using git.