A lot of times these stupid hurtful comments only affect us, and that's bad enough, but when they affect our children, that is when you really do have to draw the line. I wouldn't bother with "kill them with kindness" techniques.
To imply or actually state that you are not the real mother is something that, if heard by your child, would be detrimental to his development and to your relationship with him. To me, it is akin to child abuse and I mean that seriously. You would not let dirty old man grandpa who has a penchant for molesting children be around your child, nor would you try to "kill him with kindness." You would simply remove your child from that environment and protect him from ever being exposed to such a person. Anyone who disrupts the sacred connection between parent and child, whether because of ignorance or deliberately, deserves to be banned.
The only "kind" thing you should do is issue one warning. That you give her another chance is the kindness.