A non-Jew is forbidden from observing Shabbos. When someone is in the process of converting, we advise them to keep the entire Shabbos besides for doing one forbidden activity. A common practice is to carry something in their pocket (if there is no eiruv). However, according to some carrying is generally only breaking Shabbos on a Rabbinic level since most city streets have only the status of a karmalis (rabbinic public domain), not a reshus harabim (biblical public domain). Is it sufficient to break Shabbos on a Rabbinic level to avert the problem of a gentile keeping Shabbos? Woudn't it be better to tell them to switch on an incandescent light bulb in their room, or some other action which is simple to do and violates Shabbos on a biblical level according to everyone?
Resting for a Goy is our human intuitive definition for rest (as I understand from this source), not keeping Halachic Shabbat. I guess it would depend on that, not exactly on דרבנן vs. דאורייתא.
The prohibition of a non-Jew from observing Shabbos is rabbinic so it is sufficient for the non-Jew to observe this on a rabbinic level.