With my good friend, Brian's help, I can skate very well now.
I wonder whether this expression is right, since 'Brian's help' and 'my good friend' seem to be in apposition.
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I have had to check the definition of apposition.
According to the Wikipedia
Apposition is a grammatical construction in which two elements, normally noun phrases, are placed side by side, with one element serving to identify the other in a different way
In your example "my good friend" and "Brian's help" do not identify the same thing. I think that the apposition that you're trying to made is between "my good friend" and "Brian".
I think that your sentence is not correctly written, it should be
With the help of my good friend, Brian, I can skate very well now.
With the help of Brian, my good friend, I can skate very well now.
With my good friend Brian's help, I can skate very well now.