"There's only so much you can do. If your memories aren't intact, it's not your fault at all." Not to mention it would completely change Dominique world as she knew it. She wanted him to be able to save his best friend. Even though it wasn't possible. "I want, more than anything, for you to be able to do this. We don't really know how set in stone things are. Even if I am from 40 or 50 years into the future, who knows about parallel timelines..." Was she giving him a little bit of what she considered to be false hope? Perhaps. But it was killing her to see this man, this war hero, broken up over this. She took a long sip of her drink, fighting the feeling that she was being horrible right now. "The best thing, I think, is not to focus on the future that may come true. But to focus on your future. The decisions you're going to make."
She had no doubt in her mind that everything would play out exactly as it did before she was born. But in a worst case scenario, say Sirius did save James and Lily Potter. Uncle Harry would've grown up with parents. Perhaps Uncle Fred wouldn't have died. Maybe Teddy's parents would be around. Maybe her father wouldn't have been attacked by a (non-transformed) werewolf. Dom didn't know the repercussions of this, but just sitting around, living a life where your future was already chosen for you wasn't healthy.