The only way I think you could make decent money with Uber is to qualify for the "black car" rate -- i.e. the limo service. And then, Uber would be only one source of bookings.
For the rest, it's a part-time income supplement. I've only used Uber a few times, but the young college student working a few hours in the afternoon was the most enthusiastic. He said that he really liked the flexibility to chose when and how long to work. And, he could sit in his car and study during downtime.
A relative of a good friend drives for Uber for extra income: he saved the money for a downpayment on a condo. He still does it occasionally, and says he can make from $100 to $300 a day. But, I've always wondered if he is accounting for the gas and wear and tear on his car.
I would keep careful track of the mileage, and subtract out the IRS allowance for mileage expense from any income -- not just for taxes, but to estimate how much I was actually netting from driving for Uber.
If your friend cleared $300/day, that's roughly $75k/yr. Work 5 days a week, 50 weeks (weekends off, 2 wk vacation). I don't think they are making that much.
There is a former cabbie here on FR - interested in his opinion on this.