As far as inviting a Jewish family that one of it’s members is non-Jewish, and you are trying to get them more religious, you should ask a Rav.
AS far as the Ger in training he acquires the food before Yom Tov or Shabbos so that you are not cooking more food because of him.
This happens when you invite the elderly for a meal on Yom Tov and they have a home health aide.