Wedding invitation etiquette - can a monogram be included?

My friend is getting married and would like to include her first name initial, her fiance's first name initial, and his last name initial on their wedding invitation. She would like the last name initial to be dominant, with their first name initials smaller on either side, or something like that. Anyway, her best friend and maid of honor says that it is not politically correct to include his last name initial on the invitation since they are technically not married at the time the invitation is sent out, and my friend will not have taken his name yet. Does anyone know the correct way to go about this? Is the maid of honor right? Thanks.