When she saw him walking with another girl, she got jealous.
Literal
She [topic-は] [he [subject-が] other girl [with-と] is-walking [nominalizer-の]] [object-を] saw-and [burnt-the-rice-cake-焼きもちをやいた].
焼きもちをやく is a classic everyday idiom for 'be jealous,' literally 'burn the rice cake.' The やく puns on 焼く ('grill, burn,' as in cooking 焼きもち, a grilled mochi) and やく ('feel jealousy'). Embedded perception: she saw the event 彼がほかの女の子と歩いている and reacted.