Since she's been studying French for a full ten years now, she should be quite familiar with what French is all about.
Literal
[topic-は] already 10-years [emphatic-も] French [object-を] is-studying [given-that-のだから], French [genitive-の] what-it-is-[topic-は] well understands [should-はずだ].
何たるか is a literary/formal idiom: the question word 何 ('what') + たる (an archaic copula form, classical ~たる = current ~である) + か (the embedded-question marker), giving 'what [X] is, the essence of [X].' Used as a noun, it's the kind of phrase you'd find in essays or formal speech: 教育の何たるかを理解する ('understand what education truly is'). 10年も uses も as the emphatic 'as much/many as' — 'a full 10 years.' のだから ('given that') chains an established premise to the conclusion (はずだ), and わかっている is the resulting state of わかる ('come to understand').