A friend of mine have a DVD movie locked to a different region. Hopefully, I seem to have a region free DVD player in my laptop.
How can I burn a region free copy of his DVD movie so my friend can watch it?
You could give "K9Copy - KDE tool to backup DVDs" a try.
It's very much capable of disassembling the individual streams, putting them back together (even customising the selection of audio/video tracks) and then burning it to a new disc.
It seems it's not even capable of putting a region code onto a blank DVD, so (ab)use that as a feature!
A sort of dedicated tutorial on how to do this is on this random blog. Comes down to:
I've used Brasero for this. It's fairly trivial.
Just make an image of the DVD, then burn the image to a blank one.