She probably wants to feel better about what she's doing. If you're willing to be her friend, then she can interpret that as "See, I'm not doing anything so bad here if he's still willing to be friends." And she can tell that to family, friends, and kids to take the edge off what she's doing and imply that you were somewhat on board with the decision. And also what Uphill said...as a Plan B. If she's nervous about being out on her own, she wants to know she can call on you for help, etc.
If she's more seriously unhealthy, there are narcissistic traits where the narcissist keeps people attached (even those they leave) in case they need to feed off them in the future.
