Greg Horan thinks money is "not a big thing" for the boy band star, even though his group are on course to gross over $1 billion in 2013 through a combination of album and tour sales, merchandise and their own movie.
He told the Irish Sun newspaper: "I think he will cope well whatever happens. Niall doesn't spend that much money or talk about money, it's not a big thing in his life.
"Obviously it's amazing if the band go on to make a billion in a year, and I, for one, hope they do, but it will not change Niall. I know that much."
US website Business Insider projects One Direction - which also includes Harry Styles, Liam Payne, Zayn Malik and Louis Tomlinson - will earn $270million from their forthcoming 'Where We Are' stadium tour in 2014.
The band have reportedly sold 30 million records to date, and at an average price of $9.99, that equates to just under $300 million in gross sales.