Her "tongue" is being used to mean the way she talks to people. We are told that her tongue is "as forked as her cargo and sympathetic as a fang". To understand this we need to know what her "cargo" is.... checking google books we find that she sells live snakes!
So her tongue is as forked as a snake's tongue. The means that she lies and deceives people. And it is as sympathetic as a snakes fang. That means she says cruel and poisonous things.
This is why we have pity for the man who receives her tongue. That man is being told cruel lies.
I discover on wikipeida that traditional Rattanakosin Thai clothes were wrapped, around the body, not stitched. In formal styles this was rather like an Sari in India, but this woman seems to be have twisted and folded the cloth around her body in a more practical but less stylish way.