I live in San Jose, CA (Zone 9b). It normally stays above 40 degrees in the winter but has been known to drop below freezing for a couple hours during the night a few times in some years.
How Well Does a Desert Rose Take Cold Weather?
The Desert Rose is definately not fond of cold weather, so you may want to cover it each night or place a protective cone around it in the evenings. I personally would place a towel lining inside a protective cone for added insulation and place that on the plant each evening that low temp dips are expected even in the least of chances.