My 3 kids are all black belts in Tae Kwon Do, so it's a little different, but still a martial art. Most black belt certificates are beautiful (and oversized) and need to be framed, so my suggestion is a gift certificate to a framing shop (even Michael's does this). It's expensive to frame, but it becomes a piece of art and is something nice to display.
I've also taken pictures during the test or ceremony of my child with his instructor - you could do that and have it framed.
Is this person a child or an adult - that may make a difference in gift suggestions. I'll try to think of some other ideas.