Is this a question about "Audit Risk" The answer to that one is easy. There isn't a lot of it anymore.
But I still sign the return based on what I know and what I believe to be what is right.
And I find out what the real questions are on the return and then deal with those issues until I am sure it is ok.
The original issue here, is a simple one, Dealer or Investor. Your client did three flips. Does the client currently own other properties or owned other properties? There is no bright line test in the code.
Too keep it simple, I would put BOTH on the return. Take an amount from the gross sales, say 10% or 5% and that is the commission to do the work as a flipper on the Schedule C with appropriate operating expenses.. (SMR, Ins, M&E, etc.) of making the deal happen. And then the Schedule D has the house sales less acquisition, remodel and selling expenses.
That is what you would do if someone was in the business of doing flips, remodels, renewals or rentals.
Rich