Let's remember you asked a combined issue: "he didn't deposit the employer check into the Roth IRA due to COVID hardship,"

The employer check would be pre-tax match, and would have no basis for him. That's also why, if he had made that deposit, that would be Conversion. But you have to find out; perhaps it is post-tax. That would be unusual, but it happens. His own funds into Roth 401(k) would be post-tax.

"I believe he deposited it to his brokerage account at the same financial institution, which accepted the deposit. He hasn't rolled it over yet."

Then, he likely has passed the timeliness for a rollover (conversion). He has a taxable event; it's a separate qualification for penalty or not, and then there is the option to report it all in 2020 or let the 8915-E report it as income over three years.

"In Line 7, Part II of the 8915-E, ProConnect is not allowing me to adjust the "cost of distributions," which should be $0."

I don't know what adjust "cost" means, here. You have two 1099-R to enter, work through the worksheet, use the forms, confirm what qualifies.

"Looks like it's pulling $0 from the 2a taxable amount that was reported on the 1099-R."

Look at form 5329, to see if that applies. It has codes for you to identify the distribution is not a tax-exempt rollover.

You seem to have a need to review 8915-E, 5329, 8606.