The Mattes Method is very effective at helping people with all kinds of joint mobility issues, including frozen shoulder. If you have not yet studied the work of Aaron Mattes, I highly recommend it! Meanwhile, there may be someone skilled in this in your area (see the practitioner list on Mattes’ website) and as always, consult with your client’s doctor for clearance and/or possible PT recommendation.
Water exercise and deep breathing are also good suggestions – there is sometimes an emotional component to frozen shoulder. Side-sleepers beware: shoulders don’t thrive on that pressure all night. Converting someone to being a back-sleeper may involved support for their lower legs (atleast until all tissues crossing the anterior hip are properly lengthened), a lower head pillow with a cervical curve, and even having a buckwheat pillow on their belly to appease the ‘comfort factor’.