Oh, Im sorry, I missed RMD when you mentioned the 8915E.

For and RMD payback, use the Rollover section in the 1099R worksheet, you dont use the 8915E...thats for Covid distributions that you want to spread over 3 years.

Did they return the entire amount (adding their own funds for the w/h?)  or only the net?

You still use the Rollover section, just be sure you only rollover exactly what was recontributed.  The withholding will be considered a payment and potentially refunded on the return.