。
She seems happiest when she's with him.
Literal
She [topic-は] he [with-と] together is [when-とき] [subject-が] most happy seems.
A nested structure: いる ('to be') is modified by 一緒に, then nominalized via とき to refer to the time-period of being-together. が marks this time-period as the (small-clause) subject of the higher predicate しあわせそう ('seems happy'). The ~そう ending on a な-adjective marks visual or inferred appearance — she doesn't tell us directly, but it looks that way.