If you haven't talked to her, is it because the "Master" is not permitting that? If so, that's a huge red flag.
Remember abusers are not only physical they exert a psychological hold over the abused. They get the sub to believe that they 'deserve' what is happening to them, that somehow the sub "causes" the master to be harsh.
you have to be smart enough to know your limits and to be strong enough to draw and hold the line.
But also too as the outside party you have to be very careful to not impose your morals on top of their relationship
very very wise and true words.
BDSM can be a realization of your needs and a liberation and so finding to your own strength ... BUT there are also people out there who know how to bend it all into something destructive, diminishing and demeaning. Physically as well as psychologically. They are 'Masters of deception' and might outwardly carry the mask of "safe and sane" and sometimes they even succeed in being covered and defended by the community, and one of the hallmarks of long-term abuse in any setting is getting the victim to believe they deserve it / bring it about.