In a flashback, Mildred and Angela are seen having a dramatic argument over Angela wanting to use the car and then how Angela sees her mother as restrictive and controlling. Robbie is momentarily involved but is then ignored as the argument progresses.

Mildred also talks with her coworker, Denise.

There is also a brief moment towards the end of the film when Denise joins Mildred and others to put up the new billboards. Mildred asks when Denise was released and Denise answers (I think) three hours ago.

The argument in the flashback is about the father and the relationship between that character and francis mcdormand's character

Also, Mildred talks to her friend/boss Denise about work and about the billboards,.

However, when Denise and Mildred talk, sure, they talk about the billboards, but mainly about the police's reaction to the billboards, and speak mainly about Willoughby or Dixon. Furthermore, when Denise is let out of jail, they still talk about how Dixon was the one to let her out.

I agree with the rating. During the flashback, Angela and Mildred are talking about their plans for the evening. Mildred says she is going to grab a drink with her friend and coworker Denise, so Angela asks if she can borrow the car to which Mildred says no. Angela then calls her mother a b*tch and accuses her mother of driving her drunk while her and Robbie were in the car when they were kids as a rebuttal to Mildred's accusation that Angela has been smoking pot all day and shouldn't be driving.

Could people actually refer to the films directly when posting instead of relying on memory please? Here is the exchange between Mildred and Denise when Denise is let out of jail (multi-comment because of character limit):

D "Need a hand?"
M "Heeeeeey"
D "Ha ha haaaaa!"
M "When'd you get out?"

D "An hour ago. Judge threw it out, said the arrest report wasn't filled out right."
[They embrace]
D "Say, you..."
[Denise sees James, waves, and says more quietly]

D "Say, you didn't burn down the police station did you?"
Ja "No, she was with me the whole night."
[short pause]
D "... Why was ..."

M "It's a long story."
D "Okay. Okay."
[Jerome approaches with a billboard section rolled up]
Je "Hey."

D "Hey."
[Jerome unrolls it enough to see this section says "WILL"]
Je "Still wanna put up the Willoughby one? Him being dead an' all."
M "Why not? He paid for it."
Je "True."

Now I know some of you absolute hard-liners will say that this fails because (shock, horror) a male is mentioned (no less than three different males!) but the conversation isn't about Dixon even if his incompetance is why Denise is out. Nor is it about why Mildred was (wasn't) with James, although if it was; the topic would be the arson attack on the police station and Mildred's alibai. And it's also not about Willoughby's handling of the case nor suicide, just that the original message should remain unchanged and that Willoughby being named on the billboards now has a different meaning considering he ended up respecting the action, calling it 'like a chess-move' and calling his cooperative act of paying for more billboard time a 'counter'.

Or you could just be fucking hard-line and say that every conversation in the film stems from a rape-murder almost certainly committed by a man and that the whole film fails. But if you're of that opinion then I have to counter with the notion that the whole Bechdel–Wallace test is too interpretative to be of any use to anyone what-so-ever.