Shawn,
Use some creativity here! Send her to your jw ex-gf. Let them fight it out!
They will both be convinced that the other is in a creepy cult, and maybe they will both come to realize how similar their respective religions are!
Maybe use your family contacts to "introduce" this relgion to your ex gf as a cult. Then when she meets them, that is the perspective she will have. Once the SDA, who she is thinks is a cult member starts in, she will realize her religion is remarkeably similar.
As you said, everything happens for a reason.