I always take these claims of people being 'billionaires' with a pinch of salt.
However, make up can be very lucrative as the profit margins are huge and the items don't 'age' as quickly as clothes, which you only have one season to shfit.
However, it's also a very overcrowded and competitive market. Everyone seems to have their own cosmetics line these days. You can't just wake up one day and expect everyone to take you seriously as a cosmetics entrepreneur, after spending the past year selling yourself as a 'trad wife'. That's not to say she won't try however.