The dictionaries give this word матерщинник - person, male мат-user (матерщина - words that contain elements of мат). So this word does exist. It is pronounced as ЩИ (like usual Щ).
By the way, the female form is матерщинница. As Russian is the gender specific language, one has to realize the two forms (male/female) are often to be found for most nouns with some exceptions (names of certain professions, etc.)

I could not find any dictionary spelling it матершина. (this would be просторечный вариант; Valda, c Вашим блестящим современным образованием (кстати в Тель-Авиве?) надёжный первый вариант представляется гораздо более предпочтительным).