In all cases, they've healed the segment. That comes first, and isn't up for grabs later.
Acting under fire has a second, crucial clause that most people gloss over, which is: dig in to endure fire. When someone's acting under fire from their patient's brain, it's this second kind.
On a 10+, they're cool. Their patient's brain attacks them but they can take it, and when it's over they're unhurt.
On a 7-9, you'll have to think of something that can go a little bit wrong when someone's brain attacks someone else's. A worse outcome: "you're basically okay, but pretty shaken. Take -1forward." Or if the patient is a PC: "your brain, as you'd understand, now hates them a lot. Make your Hx with them -1." A bad choice, supposing that the angel's been keeping secrets for people (not uncommon): "their brain hooks into yours and starts to pry things out. Which of the secrets you've been keeping does it learn?"
On a miss, you'll have to think of a hard move that can happen when someone's brain attacks someone else's. You could make a case that 1-harm has been established, by extrapolating from the brainer's playbook, and just inflict 1-harm. You could capture the angel: "their brain attacks yours and overwhelms it. You're stuck inside them. You can see through their eyes if you concentrate, but it's really hard with their brain hammering on yours like that. What do you do?" You could take away their stuff: "their brain attacks yours and you feel something inside you burn out. Put a mark next to healing touch - you can't use it again until you find a way to heal it."
In short, the patient's brain attacks the angel's, and it's not part of the healing at all. It's a whole new thing to resolve.