It is illogical (and a bit daft) to think that a third person will fix what needs healing between the two of you. Not only does it just make things more complicated, it will take time and energy away from what needs to be addressed in your relationship, basically serving as a distraction from the pain, AND it lays a huge burden on the additional partner.
Has she talked to you about why she sees the relationship as "broken?" It sounds like that completely surprised you.
If I were you, I would tell her that you will consider a polyamorous arrangement if, and ONLY IF, she agrees to going to therapy or counseling with you for a predetermined period (say, 6 months) to work on healing your marriage. I would ask her not to start any relationship with that other guy during that time, after which you would be willing to discuss opening the marriage if she still feels that's what she wants. But she has no right to run rough-shod all over you. She has her terms, you have yours. If she cannot agree to that, then your marriage and relationship is not a priority for her, and I would seriously consider leaving. Sorry, but...
The world opens up... when you do.
"Oh, oh, can't you see? Love is the drug for me." ~Bryan Ferry
"Love and the self are one . . ." ~Leo Buscaglia "