Unless the client sent back the total distribution amount, including withholding, then the withholding is taxable distribution. I would show what they gave back as an indirect rollover. That would likely leave a Taxable income = withholding. 

Someday (due by May 31) the client will get a 5498 that will show what was repaid. Often they send it as a substitute https://www.irs.gov/pub/irs-prior/f5498--2020.pdf I already have one for one of my IRAs.