You'll know what to do.
I worked a total of eleven elections, from primarily elections to general elections. I even worked a special recall election where the recall was the only thing on the ballot. I was a volunteer for all of them. I worked as a "Polling Place Inspector", which means I was 'in charge' of a single polling place: I did the setup & teardown, reached out to the other polling place's poll workers to confirm they'll be there, and scheduled breaks etc..
I worked in Orange County, California, which is the county between Los Angeles and San Diego. At the time, it was very right-leaning. It may be so today, but that doesn't matter for this post.
Fun fact: In Orange County, poll workers are the only people who are allowed to question (or "challenge") a person's right to vote. The general public are not. How do I know that? Because it's one of the things I was taught during training. You can see it mentioned in [2], on page 11, under "What Are Observers NOT Allowed To Do?". (In the document, "precinct board" means "the poll workers".)
Now, three situational "pop quizzes" related to the situation from the article. In all three, you are a poll worker. Note that I will refer to procedures that were in place in Orange County, CA, not Fairfax County, VA:
Pop Quiz #1: Someone has arrived to vote, and you do not believe they are eligible to vote, what do you do?
Answer #1: You are challenging a voter. You have the voter vote provisionally. Their ballot would be sealed in the envelope, and their information (plus an explanation of why you're having them vote provisionally) would be on the envelope. The challenged voter would take a receipt with them, giving them a phone number to call, should they want to check up on the status of their vote after the election.
Fun Fact: Challenging a voter without probable cause is a felony in the State of California. How do I know that? Because it's in the instructional handbook that every poll worker gets, when they go through training. You can find Orange County's handbook for the 2018 election at [1].
Pop Quiz #2: Someone at the polling place, who is not a poll worker, is challenging peoples' right to vote. What do you do?
Answer #2: Call the dedicated polling place helpdesk, letting them know about the incident. Depending on the person's behavior, you may ask them to leave, or you may skip directly to calling the police. Your polling place inspector would have already looked up the phone number of the nearest police station, or you could just call 911.
Fun Fact: As part of polling place supplies, I received a county mobile phone. I was specifically instructed to charge it up in advance of election day. They were always chunky Nokia phones, which felt like they could be used as a weapon in an emergency.
Finally, to address your question…
Pop Quiz #3: Another poll worker is challenging a voter, and you believe the challenge is unlawful. What do you do?
Answer #3: If you are not able to dissuade the poll worker into allowing the voter to vote normally, then you have them vote provisionally. The most important thing is to get the voter through the process, and their provisional envelope into the box. Once that is done, you reach out to the polling place helpdesk, letting them know who did what.
Indeed, quoting from the article you linked, "After the [polling place] manager intervened, Burrell-Aldana was allowed to vote." The article does not say, but I expect the polling place manager was already planning on how to communicate the incident back to headquarters, and was keeping an eye on that poll worker.
If you had volunteered for this election, and you happened to be in the situation from this article, then you would have known what to do. :-)
[1]: https://ocvote.gov/fileadmin/user_upload/elections/gen2018/T...
[2]: https://ocvote.gov/election-library/docs/Election%20Observat...