Git merge from someone else's fork
Add their github fork repo as a remote to a clone of your own repo:
git remote add other-guys-repo <url to other guys repo>
Get their changes:
git fetch other-guys-repo
Checkout the branch where you want to merge:
git checkout my_new_branch
Merge their changes in (assuming they did their work on the master branch):
git merge other-guys-repo/master
Resolve conflicts, commit the resolutions and voila.