I get the sense in my local BDSM community that contracts are similar in how many people here feel about rules. In other words, they are helpful to have starting out but that things change greatly over time, and as things go on and the relationship develops, the rules - or the contract in this case - changes too, or goes away entirely as trust is earned on both sides. However, others continue to find contracts really useful even though they have no legal basis at all.
I have never heard of a legal situation where a BDSM contract was of any relevance or use. Usually I hear about situations where any acknowledgement of BDSM interest or participation was actually detrimental (such as liking pain means consenting to getting beat in all situations). But I certainly don't know the entirety of the BDSM universe.
@pollyanna, how dreadful for you and your friends. I hope that pathological user is named among all of your acquaintances, and not just the BDSM ones. Someone like that will abuse people in all areas of life - calling one self a mistress is just the means to the end. Even if you and your friends had intervened, she may not have left. Abused people often stay with their abuser for a variety of reasons.
It is so important that one have a strong sense of self in BDSM. That's true anywhere but BDSM has real psychological and physical dangers. The pyschological dangers are more pervasive and threatening in my opinion because one can be relatively easily taught to swing a whip safely. It's a technical skill. The moral, emotional or pyschological skills are much harder to develop - they require empathy, love, and care be already present. One can be taught how to be an ethical master, top dominant, submissive, switch, or slave. But someone without ethics, or compassion or empathy, like the sociopathic mistress, can mimic a 'good' top or dom by picking up the technical skills and just enough to pass on the pyschological skills.
I have met a lot of secure, strong submissives and slaves locally. I have also met a few submissives or slaves who so lack a sense of self that they felt like a sucking black hole to me. I've met a few who were desperately needy - which made them desperate to please, no matter the cost. Your friend perhaps sounds like these people. Her 'mistress' is responsible for that tragedy. No question. That was a classic controlling and abusive relationship with M/s as the mechanism for the abuse.
But I also strongly feel that BDSM, like relationships in general, should not be engaged in if one is not psychologically healthy enough to do so. And I see some people locally who do not meet that standard, in my opinion. And unfortunately that makes them victims just waiting to run into a user and abuser.