Hi,
How you carry out the treatment really depends on the severity of the psoriasis. As sufferers often have dry, loose skin, you may find that the sugaring removes more than just the hair, and may cause the top layer of skin to be removed that will result in grazing.
The best course of action would be to avoid as much of the affected area as possible, so as not to cause your client any discomfort and avoid the risk any cross infection if the skin is open. If in doubt refer the client to their GP for advice prior to having the treatment.