“doesn’t even know what Shabbos is”
experience has it that they DO know what Shabbos is, and all the learning/mitzvos they had before are still with them
the best thing for them (and you too) is to keep on being friendly to them, take them out to ice cream, etc.
often times when ppl go otd r”l it’s because they feel isolated. if you show them the true colors of a friendship, they’ll go back to being frum simply by osmosis
hatzlocha !