I interpreted that question as who are real economists that are not Keynsians, since you had asked, in the interim, if a real economist must be a Keynsian and I responded no, that Chicago Schoolers count, but Austrian Schoolers do not.
That said, let's go ahead use Friedman's 23% income tax as a baseline for what is realistic.
The most recent flat tax proposition that gained any widespread national appeal was Steve Forbes' in the '96 and 2000 election cycles, and he suggested a rate of 17%. A few senators floated a plan about a half dozen years ago or so, and I think that was around 15% for its base rate. Both would have exempted everything but wages from the tax, meaning capital gains, dividends, and interest would be taken tax free. Both would have exempted a certain amount of money from any taxation.
A 2010 piece from US News and World Report shows that the Cato Institute likewise still uses the 17% figure. I have not seen a modern flat tax proposal of any significance that uses Friedman's 23%.
Therefore, I think it's fair to say that even Friedman would find those plans unrealistic.
The so-called Fair Tax is understated in so much as proponents call it a 23% tax instead of the 30% that it actually is. To that extent, I suppose they have a realistic number (30%) that they're trying to sell as being smaller than it really is to try to make it more palatable.
However, since not all income is spent, to be revenue neutral the sales tax would have to be a higher rate than the associated income rate. Since Friedman supported a 23% income tax rate, the comparable sales tax rate would have to be at least somewhat higher than that. Indeed, at least one political watchdog, FactCheck.org,
says that the 23% inclusive figure is still not high enough. It's not grossly understated (FactCheck, citing the President's Advisory Panel on Tax Reform, figures it'd take a rate of 25% inclusive or 34% when treated as a traditional sales tax), but understated nonetheless.