、。
Yumi is one of my friends.
Literal
Yumi [topic-は], me-of friends-of one-person is-[polite-です].
友達のひとり ('one of [my] friends') uses の + counter to express 'one of a set.' Note the comma after 由美は — standard in written Japanese for pause and clarity, especially when the topic is fronted for emphasis. 友達 can be singular or plural; の here makes it clearly plural.