It's probably not the best time, but so long as you get all the roots and keep it well watered you should get away with it.

If its a large established bush you will probably need to prune the top to limit the stress on the plant.
Better really when its dormant, I,m assuming its a hardy variety planted in the ground

Why not take a few cuttings to be sure? They root quite easily and if you finish up with too many they are easy to give away! Do you know what variety it is?