。
She was excited because he was there.
Literal
She [topic-は] he [subject-が] existed [because-ので] [excited-わくわく] was-doing.
わくわく is a 擬態語 (state mimetic) for excited anticipation, an inner thrill. Combined with する (here as ~ていた, ongoing past) it functions as a verb: 'be in a state of excitement.' The ~ので gives the soft reason — 'because he was there, she was excited.'