There's a part of her, the part that's seen just how ruthless the Doctor can be, how much he needs someone to stop him, that can imagine there's someone out there who feels like Callisto. The Doctor is a comet, and Donna has seen some people get burned in his wake. She might be one day... but she hopes that won't come. He's a good man now. He's trying to be one, and that has to count for something. But it's not her place to tell this woman to forgive someone who did something so horrible to her.

"I'm sorry." It's said earnestly. "I know that doesn't really do much for you, but I am sorry you've gone through all that. I can't imagine how hard it must have been." She isn't even going to get on Callisto about ripping up a good towel.