I think "since I knew her" sounds off in this sense (at least in my particular Anglo-Irish dialect).
Unless 'since' is being used as a synonym for 'because' (or 'knew' is being used "in the biblical sense", which is popularly taken to mean "had sex with her"). :-|
The reason is that 'know' is neither an action verb nor something that happens at a discrete moment of time. It is a disposition. And it's one that the speaker clearly still has (since they're speaking about her).
There is of course the comic/tragic song (https://en.wikipedia.org/wiki/Johnny_I_Hardly_Knew_Ye) but that sense is from an old Anglo-Irish dialect and means more or less "recognise."
For the meaning you describe, I'd say most native speakers of standard or near-standard English would put something like "last met/saw/spoke to" where you put "knew".
The other (possibly more important) problem with "since I knew her" in your sense is that it's far too easy to misunderstand the speaker; thinking they mean "since I've known her."