Note that if the boundary is being crossed, you have to remove yourself from the situation, and that's the part you don't seem to be carrying on, since you are still on the receiving end of it.
But I'm unsure what you allow or disallow. It seems like your boundary is about her not having sex, since you said it was ok for her to meet the first W. If she didn't have sex, doesn't that mean that she hasn't crossed your boundary?
Anyway, the fact that you have shelters in your country is a great advantage, and it takes from you a lot of the responsibility. She will survive. If you wish and she accepts, you can give her emotional support throughout this process by visiting her in the shelter, and that could have a positive impact on her outcome. I hope that makes you feel less guilty.
Some clarification- there really aren't shelters in the US for domestic disagreements and there are no public ones. There are some limited ones for women in domestic violence situations. These are privately run, spaces and availability are limited. Legally, in the US, Max does not have the option of putting his wife out of the house. Neither can he just up and leave as this would be considered legal abandonment. The only real option is legal separation.
IMHO, for Max- you are more in a reactive position. You have a boundary according to what you want, but when it's crossed, then what to do about it isn't so clear, because I really think you don't want to leave.
This isn't criticism- I observed this with my parents. My BPD mother would do something that to me, was unacceptable. Dad, not wanting to leave, would somehow rationalize it.
It's a challenge to act on a boundary when you don't want to, and are married to a spouse who stretches the limits of them. However I think even pwBPD have a sense of what is too far. This is why, I have a hunch that Max's wife may have started something with another woman but may not have "gone too far" with the relationship. It's possible she may have self sabotaged it.