。。
He's good at French. English even more so.
Literal
He [topic-は] French [subject-が] is-good-at. Let-alone English [topic-は] all-the-more is.
Note the subject is 彼 ('he') here, not 彼女. 得意 ('good at, strong in, one's specialty') is a な-adjective marking skill domain — paired with the が-with-skill-predicates pattern. The closing なおさらです again leaves the predicate '(is good at)' implicit, with なおさら marking the implicit claim about English as 'all the more so.' A standard rhetorical extension structure.