Shouldn't 'clarifying' be acceptable as well?
It's an a bit surprising answer to me, but I guess it's okay. Added now.
It's a bit of a surprising answer. English US Native.
My I explain that I'm a friend of theirs was wrong
Added "a friend of theirs".
I'm making it clear that I am their friend, be acceptable?
Is there a reason for the comma?
It introduces the second clause. In 99% of the cases, "że" is preceded by a comma.