I know Uber drivers count driving/car costs as business expenses for a tax write off. Something to think about...
Lyft drivers could surely benefit from the tax deductions as well. I assume they report their Lyft payments to the IRS? This complicates the "how much they earn" formula further.
It would be exceptionally unwise to fail to report income which has been 1099-ed to you, and very unwise to fail to report substantial income merely because one does not have a corresponding 1099 for it, because (particularly in the case where one party has all the information required to reconstruct your income and that one party can simultaneously uncover millions of dollars of under-reporting) audits exist.