"I appreciate that it might be a little tricky to distinguish whether a relationship is a consensual one or if it's abusive, especially in a relationship of this type. And I'd be very hesitant myself about sticking my nose into an intimate relationship. But since this is bothering you enough to bring it to the attention of the members here I'll suggest that you should respect your intuition. I think you should at least consider the possiblity that this guy may be an abuser. You mention that this "Master" says he really loves his partner, and that his treatment of her is consensual. Do you believe him? Have you talked to her? What does she say? If you haven't talked to her, is it because the "Master" is not permitting that? If so, that's a huge red flag. I'm no expert, but I understand that there are several relatively consistent signs of an abusive relationship. Since you're clearly worried about her, I would encourage you to look into it. If this is an abusive relationship, the abuse could escalate."
Oh boy this is always a tough one because what one person terms as abuse is, honestly, another person's foreplay.
My Mistress and I have a consensual relationship where under the appropriate circumstances she treats me like her slave and under others she treats me like a partner she loves (which she does).
I have always drawn the line at the use of the "safe word". I can (and will) take a lot of punishment, I don't want to know what a Dom(me) is going to do to me, I don't want them to ask permission.
What I insist on is being able to trust that the person I am being submissive to that if they hear, or even think they hear the "safe word" everything comes to an immediate screeching halt until the situation is clarified. If they don't stop when the "safe word" is used then they have crossed the line into abuse.
No ifs ands or buts.
So if I was confronted with this situation, I would get together with the sub (without the "master" knowing about it) and ask a direct question. And that question is: Do you have a safe word and has he ALWAYS respected it? If the answer is no or even if there is hesitation you probably have an abusive situation.
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. Unfortunately too many people who are not internally strong get themselves into these situations.
If you are going to be a sub you have to not only be able to endure pain 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. My mother would be horrified if she knew my life and what I enjoy. My oldest brother understands so he doesn't interfere but if he thought for one minute I was being abused I have no doubt he'd break (and I mean that literally) the person abusing me. He always asks, and he will always be my escape route if I ever need one, but he won't interfere in my lifestyle choices. He tells me I'm an adult and I have to live with my choices but he also says I have to be strong enough to ask for help if I need it.
It isn't easy no matter how you look at it.
I hope that helps.
kisses