It will not revert to the upright cultivar. Cut it all the way back to the ground (6-10 inch stumps). I would do it in the later spring right before new growth is ready to emerge. I also suggest that you do NOT fertilize this plant for the upcoming season.

The root system present in these plants have been produced to support a much larger plant body than what will remain after the 'procedure'. Thus, you and your client will see an amazing amount of new growth...fueled by all of the stored energy in that large root system. Fertilizing will only tax the plant at this time.

By the way, this is called 'rejuvenation pruning', and is an acceptable (and commonly done) method of bringing over grown woody plants back under control.

In subsequent years, do the proper method of pruning to maintain a weeping shrub in pretty form.