。
She bumped into an old friend by chance while walking through the park.
Literal
She [topic-は] park [path-を] is-walking time accidentally old-time 's friend [with-と] met.
出会う ('to encounter, meet, run into') is more event-flavored than 会う ('meet'); it carries a sense of discovery or fateful encounter. 偶然 ('by chance, coincidentally') is a stock adverb for unexpected encounters, placed before the verb. The temporal frame ~ているとき ('when X-ing') uses 時 to fix a moment within an ongoing action. Particle work: を on 公園 marks the path traveled, と on 友達 marks the encounter partner. Bumping into someone in the park is a common dorama device for kicking off plot or reuniting characters.