The satellite would raise tides in the primary and vice versa. The tides would dissipate rotational energy via Sedna-quakes. Over time, the rotation rates of Sedna and its moon would be such that each would present the same face to the other. So since Sedna's day is 20 earth days long, we might expect a moon with an orbital period of 20 days.

If there's no moon, you have to lose the angular momentum Sedna had when it formed somehow.