。
She had been looking forward to seeing him again.
Literal
She [topic-は] him [to-に] meet-again [nominalizer-の] [object-を] enjoyment [as-に] was-doing.
The pattern Xを楽しみにする ('look forward to X, anticipate X') is the standard idiom for joyful anticipation — 楽しみ ('pleasure, anticipation') + にする gives 'make X into one's anticipation,' and the ~ていた past continuous marks the sustained anticipation in the past, often leading up to a moment when it was either fulfilled or interrupted. Here the embedded clause 彼に再会する ('meet him again') is nominalized by の and made the object of 楽しみにする.