It sounds like he saw an opportunity to secretly influence a vulnerable woman and pounced on it, and is now involving her in sexual and BDsm situations for his own satisfaction, not hers. I am not into BDsm but have read enough to know that the dom is supposed to do everything they can for the benefit of the sub, not for the dom's own self-aggrandizement. It is unconscionable that he would pressure her to have sex with someone she doesn't want to or force you to be part of their dynamic - especially considering you have not consented to polyamory yet! How dare he do such a thing, and covertly! He is a shithead, plain and simple. Do they have a written contract between them?
She's making bad decisions and going about this all wrong. I fear it's because she's been depressed and this asshole guy is exerting pressure on her, trying to sell his snake oil. My ex would say someone should throw him a beating. I would get into therapy with her as soon as you can, this looks like a dangerous situation. There are poly-friendly professionals, and many who specialize in alternative lifestyle counseling are also a good bet.
Some links for you:
Polychromatic: Poly-Friendly Professionals.
Loving More's List of Poly-Friendly Professionals.
Psychology Today: Find a Therapist
. With this one, you can select "Relationship Issues" after you narrow it down for your zip code.
As a last resort, you could sign her into a psychiatric hospital or ward, citing that she's become a danger to herself as long as you can show proof that she put herself in harm's way. As her husband, you could do that. This is NOT to say that BDsm is a mental illness; I am simply responding to her depression and isolation, followed by a sudden shift in personality, and then letting someone she doesn't know well start controlling her life. Hospitalization isn't as bad as it sounds. I've had to intervene in that way for a close relative many times and it served them well. Sometimes people need a break from daily living to get better when their heads are not in a good place.
You could also ask her to read some good poly literature with you. There are a few good checklists to discuss with her over at Tristan Taormino's website (she wrote Opening Up
, a good book to read together) here: http://openingup.net/resources/free-...om-opening-up/
. You can also invite your wife to visit this forum and add her point of view, so we can help her and share some better options with her.