I don’t have practical advice for you. However, a general attitude that can help is not to classify them as another type of person. There is no Chiyuv to be Frum. There is a Mitzva to don Teffilin, to keep Shabbos, Kashrus, Succah, Tzeddaka, Krias Shema and so on.
You don’t have to try to make them ‘Frum’. They already are full fledged Yidden. You can convey to them this outlook that they never ‘left’, they merely slowed, or even stopped doing several details. In this vein, they can make Brachos when eating at your table, or Daven, make Kiddush, shake Lulav, and anything else. They don’t need to feel threatened, since they aren’t making a statement of becoming Frum.