Suggest that you take a look at page 31 of the preamble about why guaranteed payment for the use of capital under §707(c) is not attributable to a trade or business even though it part part of the flow-through.  There are people who argue in response that it should be but take a look at pages 45 and 46 why that position is not accepted.  How then would you argue that flow-through for charitable contributions, which are determined without regard to the income of an S-corp or partnership, are attributable to a trade or business.

Of course, the IRS would likely not reject the return where a position is taken to reduce the QBI for charitable contribution flowing through a K-1 but your client would be disadvantaged because of that.