Well.... my advice and what I would do are two different things. What you should do is stay away, don't text her any more and focus on you and the kids. By contacting her you're getting in the middle of things and the best place to be is outside of it because it's not a healthy place. Their relationship, the whole thing is ugly and it's probably best not to get tangled up in it. Being the strong, safety net outside of it seems like a much better place to be.
Also, trying to convince her not to get involved with your husband is probably going to have the opposite effect. It's like trying to tell a teenager not to date a certain kid... then they just want to do it more!!!
As far as what I'd do (and have done!) in a similar situation... I can't go there. It's not the best advice because there's lots of potential for backfire and the chance of creating enormous havoc. You only go there if you're willing to deal with the consequences.
There is no arriving, ever. It is all a continual becoming.
