。
Because she was a foreigner, she was treated as a foreigner.
Literal
She [topic-は] foreigner was [because-ので] foreigner [as-として] was-treated.
Three useful pieces stack neatly here. ので after a clause gives reason or cause — slightly softer and more 'matter-of-fact' than ~から, and preferred when you don't want the reasoning to feel pushy. として attaches a role or capacity to a noun: 外国人として = 'as a foreigner / in the role of a foreigner.' And 扱われた is the passive of 扱う ('to handle, deal with, treat'), so the whole second clause reads 'was treated as a foreigner.' The sentence has a tautological flavor — 'because X, treated as X' — and would typically appear in commentary on cross-cultural treatment or expat experience: the implication is often that the treatment was different from how she might have been treated otherwise.