I hadn't heard of a  338(h) election because I don't generally deal with M&A clients.  A starting point is here, but it does not explain what happens to the 20% lagniappe:

https://www.rklcpa.com/338h10-transaction-structure-pros-cons-sellers-buyers/

Maybe because it's obvious that if you only sell 80% of something, you still own 20% of something, and both IRS and FinCen will want to keep in touch.  

Don't be afraid to tell your client, "you got yourself into this mess, find someone else to help you get out of it."