I agree with Deniz that you include 100% of the numbers on the income statement and the balance sheet, and that only the GILTI and Subpart F Income inclusions are computing based on the reduced percentage.
If the client sold shares of the CFC at a gain, be sure to consider section 1248, which may cause all or a portion of the gain to be recharacterized as dividend income.
Since his ownership did not drop below 10%, I don’t think the client needs to file as a Category 3 filer. I also don’t think a Form 926 would be required, regardless of whether the purchaser is foreign or not.