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[i] Do you know any friends of hers?
[ii] Do you know any friend of hers?
The first is from a Korean grammar book and the second is mine. Whenever I come across any in questions, it’s very tricky to understand. From OALD #1, the first seems to imply that there is a set of her friends and ask if the listener knows at least a few of them. And the second question seems to ask if the listener knows her friend, whoever they are. Is my understanding proper?
Both are technically correct and mean the same thing.
The first sentence is more widely used, asking if you know a person she also knows. Although 'friends' is plural, the use of 'any' means 1 or more.
The second is more formal sounding, but means the same. The first, I would say, should be used.
Very often we ask questions in plural form, whether we expect the answer to be singular or plural: 'Do you have (any) children?' or (closer to your question) 'Have you read any books of hers?'. You can answer even if you have only one child, or have read only one of her books (or indeed if you have no children (note, not 'no child') or haven't read any of her books).
The second usage is not correct because there is a number disagreement: "any" refers to a set, while "friend" is exclusively singular.
If there is a specific, singular friend being asked about, it should be,
Do you know her friend?
Alternatively, if the speaker wishes to ask about a singular, but unknown friend, they would probably say,
Do you know anyone who is her friend?
