No, you don't use mustard. That's cruel and stupid. Are you insane? Is this how you treat a baby? Putting mustard in his mouth?
Hysterical much? Jesus, it isn't like she's suggesting putting something poisonous on her boobs.
OP- putting a bitter substance on the nipples is a traditional weaning method that was suggested to me when it became clear that my 3 YO had no intention of weaning. I tried putting soap on my nipples, but it had no effect on him at all.
If I were you, I would try it along with some of the other weaning methods suggested here. My son finally got with the weaning program when he was closer to 4.