It seems to me that there alot of issues in play here. Her depression and declining self esteem made her a perfect subject for a domineering (not dominant) and manipulative connection with this "friend" who'd come out of a bitter divorce.
Over the years and at the many BDSM events I've attended in the US - I have witnessed first hand abusers and manipulators wearing the guise of "dominant" and they do not take submissives so much as hostages. Their need to control and micromanage is to my mind disturbing. And the relationships of this kind are abusive and damaging physically and psychologically. A dominant with integrity, poly or otherwise would not take on a submissive who is in a diminished emotional state NOR would they insist that submissive have non-consensual sex with another. A sincere dominant would make sure a submissive gets the care they need before proceeding with any deeper emotional or sexual connection. A sincere dominant might have contacted you to express his concerns and offer to help in finding your wife the care she needs before proceeding to anything else.
Your wife's depression needs diagnosis and treatment because until that is addressed, how can the veracity of her dicissions be determined? If you determine that she is a danger to herself by being in an abusive relationship then perhaps an intervention could benefit her. Judging from your message, you are a nuturing partner dealing with a dizzying number of issues simultaneously. Remember to take care of yourself too. All the players in this situation are adults and need to take responsibility for their actions. If a medical intervention is warranted for your wife and you are alright with that, fine. If not you need to let her go, with love. Let her go do what she needs to do; let her know you love her but you will not accept living in this situation any longer. Knowing and establishing your own boundaries and limits - be clear about under what circumstances she would be welcome back in your life if at all. The relationship with your wife is not a one way street; it is not imperative you to do all the care and work in the relationship. At some point she will have to accept responsibility for her own decisions.
Sometimes not making a decision is more painful than finally taking action.