"Shark Tank" investor Kevin O'Leary is impressed with how Los Angeles-based millennial Graham Stephan is handling his money. The 30-year-old real estate investor and YouTube star earned more than $1.6 million last year and saved about 99% of it. 

O'Leary particularly likes how Stephan, who became a millionaire when he was 26, is aware of his expenses and, despite being such a high-earner, still saves money on things like coffee. (Stephan makes it at home for about 20 cents a cup.)

"He knows exactly what he's spending his money on," says O'Leary, who also refuses to spend at coffee shops. "He's very, very conscious of not wasting a single cent." And he's figured out how to earn a lot of money through various income streams, which O'Leary respects.

"The kid's a hustler," he says, reacting to a Millennial Money episode featuring Stephan. "He's smart, he's a hustler and he stays focused on what he has to achieve. Those are attributes of a great entrepreneur."

While Stephan does a good job earning and saving, he could work on diversifying his investments, says the "Shark Tank" star. Stephan's main focus is real estate. He owns and rents out six properties in L.A., including the duplex where he lives, and he is looking to buy more homes.

"There's nothing wrong with doing what you're great at," says O'Leary, but "real estate is just one sector of the economy and it goes through cycles, too. ... It's very prudent to put something aside and just buy [into] the overall market, so you get diversification of your net worth." 

Stephan can buy into the overall market by using index funds, which hold every stock in an index such as the S&P 500, including big-name companies like Apple and Amazon. That way, "when real estate slows down, you have other investments that can continue to perform," O'Leary says.

But overall, Stephan is on the right track: "On a scale of one to 10, with one being a horrible saver and 10 being brilliant, I'm going to give him an 8.234. He wasn't a 10 because he didn't diversify enough, but he's really good at staying focused on saving and making money."

Watch O'Leary's full reaction to Stephan's spending and savings habits. 

For more tactical advice from Mr. Wonderful, watch Money Disputes premiering today on CNBC.com.

Don't miss: How a 29-year-old YouTube millionaire making up to $220,000 a month spends his money

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