Maybe she went back with the Avengers that were there? Weren't there Avengers who ended up down there in one of the Avengers Tie- Ins?
Yeah, she made a brief appearance in Avengers #15 when Spider-Woman, Ms. Marvel, the Protector, and Hawkeye went down to Brazil to fight Nul/ Hulk. When those four were called back to New York in Avengers #17, she was no where to be seen.
Three of those four made a brief appearance in Ghost Rider #3 on their drive back to the states (their Quinjet had been destroyed in the battle with Nul). They were in Columbia at the time. Since the Protector isn't seen, it's POSSIBLE that Red She-Hulk is also just off panel (more likely the writer and/ or artist on the book just forgot about the Protector).
All of them are back in New York for Fear Itself #6 when the teams split up to take Thor back to Asgard and to defend New York. Red She-Hulk is no where to be seen.
And the problem I have with it is that Fraction KNEW where he was going. He knew that he was setting up specific Avengers to become the Mighty because we saw teaser images a LONG time ago. Maybe there were last minute changes. Maybe. But still, the series doesn't set up the specific characters in Broxton in the story. Red She-Hulk is just the biggest offender but not the only one.
In Fear Itself #6, Luke Cage and the New Avengers are left to defend/ evacuate/ whatever New York City. That places Iron Fist and Dr. Strange in NYC instead of Broxton and no panel in FI #6 magically shows them traveling to Fallen Asgard. Instead, Hawkeye specifically says that Luke's team has New York while they do their jobs here. Spider-Man is still swinging around New York, looking for his Aunt after Rogers and his team teleport to Asgard.
The Avengers at the World Tree as seen at the end of Fear Itself #6 are Captain America, Red Hulk, Ms. Marvel, Spider-Woman, Protector, Hawkeye, and Wolverine. We see Dark Asgard floating in the background of that issue. There's no time for the others to make a crosscountry trek.
But in Fear Itself #7, there they are. No explanation. They're just needed for the plot so there they are. It makes me wonder if Stuart Immonen and Matt Fraction were really on the same page for what they were planning or if Fraction was just making it up as he went along. It's sloppy storytelling. I keep thinking I might be nitpicking but this isn't that. This is "Character A was not in position for his next scene so suddenly he was." With seven issues to work with and a multitude of tie-ins, it should have been an easy matter to get the right Avengers in the right place and have that make sense. Instead, it seems like random Avengers are drawn in every scene without anyone checking the plot.
But I read all of the Fear Itself issues and then wrote about them so I'm much more burnt out than your average reader who probably didn't notice that Spidey was still swinging around New York when last we saw him. I get that. It's still sloppy storytelling.