Yep. Just because you know them in one guise, doesn't mean they are satisfied in that guise. Just look at Type-A dominants in the business world who need a release from the stressors and find it through submission... or at least bottoming.

You may never get a glimpse of that other side of them. Consider yourself lucky your friend is so open with you. I suggest supporting them in their search (so long as it doesn't impinge on your own needs nor your mutual friendship.)