Hey, I don't know if you're still looking for an answer, but I have one.

When Violet notices the trajectory of Solas' fire will hit her squad (likely intentional, she thinks) she shouts for everyone to get down. Here's the relevant part of the story (hardcover chapter 8 page 70):

"Get down!" I shout, lunging for Sloane and tackling her to the ground as fire blasts overhead, the flames so close that heat singes every patch of exposed skin on my body. 
To Sloane's credit, she doesn't cry out as I cover as much of her body as can, curling over her, but the soul-rending screams behind us are unmistakabl. I open my eyes long enough to see Aaric laying flat over the redhead under the endless stream of fire. 
Tairn's roar fills my head as lava licks along my arched back. 
A scream musters at the base of my throat, but I can't breathe in this inferno, let alone give it voice. 
As quickly as it struck, the heat dissipates, and I fill my lungs with precious oxygen, gasping for breath before shoving off the gravel to my feet. I turn to face the aftermath as the other second- and third-years around me rise.

In the thie sixth paragraph of page 70 it says "Tairn's roar fills my head as lava licks along my arched back" (third paragraph in the above quotation). My best guess it that this is simply supposed to be a word to represent the pain she feels. It clearly states several times on page 70 that it's flame coming out of his mouth, not lava ("flames churning up his throat," "as fire blasts overhead," "the flames so close," etc.). So it's just something Yarros used to try and give words to the pain Violet was feeling.