"She thinks/calling the distribution a "rollover""

They can call it Chickens. That doesn't mean it Clucks.

Take everything sequentially, and by Type of IRA, because the rules matter.

DOD 2020. That means inherited Traditional or Roth? And not by Spouse. And that means SECURE Act applies.

Then, what was the next event? Let's break it down:

"Her share of the IRA is in her name and name of deceased."

What "share" and how does that apply?

What is this account (existing or new, Trad or Roth) and how is it titled (owned) as Deceased and Beneficiary, or as?

"Advisor also created a Roth IRA in the name of the taxpayer so Trustee to Trustee. DOD was Dec 2020."

Created the Roth, when? The "taxpayer" is the Beneficiary you are working for? The Date of the first deposit to the Roth is when? Where did that money come from? What happened next?

"$8,000 was distributed in 2021 with 1099-R:"

Distributed from the Roth?

"Line 1 = $8,000 Line 2a $8,000 Line 2b Marked X for Taxable amount not determined"

Do you know how long the deceased's IRA account existed? If that is/was also Roth, this matters.

"Withheld for Fed & State Line 7 = 4"

4 what?

"No contributions and only $8,000 distribution, 2021 RMD Basis Value = $50,104"

"Advisor also provided statement that $6,000 Roth IRA contribution was made in 2021 in taxpayer's name."

You just stated No contributions.

From the IRS: "If the inherited traditional IRA is from anyone other than a deceased spouse, the beneficiary cannot treat it as his or her own. This means that the beneficiary cannot make any contributions to the IRA or roll over any amounts into or out of the inherited IRA."

https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-beneficiary

Notice that states traditional IRA. That's why we need to know which type of IRA the deceased owned.

"Roth Account Value as of 12/31/21 - $5,978.92."

You already told us a Basis was over $50k.

Are we mixing two or more accounts into this discussion?

"Based on these facts, is all the $8,000 taxable?"

It's not clear, yet.