、。
The moment she found him, she burst into tears.
Literal
She [topic-は] him [object-を] find [the moment-やいなや], suddenly started-crying.
Three notable features. やいなや (や否や) is a literary/written 'the moment that, no sooner than' connector — attaches to a verb's dictionary form and means the next event happens immediately, almost simultaneously. わっと is a 擬音語/擬態語 for sudden bursts — typically loud crying, shouting, or rushing. 泣き出す is a 複合動詞 (compound verb) combining 泣く ('cry') + 出す ('begin/let out'), where the auxiliary 出す turns the base verb into 'burst out, suddenly start.' Together: the dramatic, almost cinematic version of 'she found him and started crying.'