It’s not before she shoots him, it’s after. She knows, because in 1977 she shot a man that looked exactly like her now-grown-up son, and she understands that time is immutable.

> So, everything that the losties are doing in their “present” always happened in the hostiles past, it wasn’t necessarily the losties doing it but it did happen, for example Ben was probably always shot, and always healed by the hostiles, but in circumstances that didn’t involve the losties. Make sense? No? Didn’t think so.

The way I understand it is that everything that we see happen, happens, and there is no alternative. There was no alternative in which Ben was shot by someone other than Sayid, no alternative in which Kate and James were not the ones that brought Ben to Richard. There certainly cannot be an alternative in which Locke was not the one that talked to Richard in 1954, otherwise why would Richard go to see Locke growing up? As you pointed out, the piano scene with Ms. Hawking and Dan seems to suggest that she always knew what happened to him, not that she (or anyone besides Desmond) just woke up with a memory in 2007. Widmore tells Locke that he first met Locke when he was 17, not that he woke up with a memory a few months ago, in which Locke came to their camp.