。
She bought every one of us an ice cream.
Literal
She [topic-は] us each-one-each [to-に] ice cream [object-を] treated-us-to.
おごる ('to treat someone, to pay for someone') is a casual social verb central to Japanese eating-out culture — when 先輩 take 後輩 out, the senior typically おごる. Combined with ~てくれる ('do for me/us'), it foregrounds the speaker's gratitude: she did this kind thing for our benefit. 一人一人 (literally 'one person one person,' read ひとりひとり) is the distributive 'each one individually' — making clear that everyone got their own, not a single shared ice cream. Reduplication (~一~) like this is a productive way to form distributives: 一日一日, 一つ一つ.