At least since it’s a girl marrying a Goy the kids will be Jewish.
Every person has to decide what’s right for them. If you are physically/emotionally capable of remaining close to such people then it is a great thing if you can keep their connection to Yiddishkeit. But if it harms you (either spiritually or emotionally) then maybe you need to consider not being in contact with such people anymore. You can’t live your friends’ lives for them. All you can do is be there as a support and hope that they eventually return. But a support that it itself bends or breaks does no one any good. From your post it sounds like maybe it’s not your Tafkid to do Kiruv like this. Do whatever you can but be conscious of your own limits. If you try and break your own emotional limits, even though you have the very best intentions at heart, you might end up harming yourself more than you help anyone else.