。
She's my only friend.
Literal
She [topic-は] my sole friend is.
唯一 ('sole, only') is an absolute modifier — there is exactly one. Stronger than 一人だけ ('just one person'), which counts but doesn't necessarily exclude others by definition. 唯一 carries weight: it implies the speaker has no other friends, which gives the sentence a slightly poignant tone. The Sino-Japanese 唯一 is often pronounced ゆいいつ; an older alternate reading ゆいつ also exists. 友達 (vs more formal 友人) keeps the register conversational.