Student or Learner
It seems always strange to me to see 'some friend' ('some' used with a singular countable 'friend').
But if it is correct, will my explanation below be correct?
'I have a friend who likes it'- I know who I'm talking about.
'I have some friend who likes it'- I don't remember who in particular, but I remember that he/she likes it.
In BrE we would say, " I lost my pen somewhere or other." "I gave my pen to somebody or other." "I was going to note down some item or other (on the shopping list) before I lost my pen."
"I was going to call some friend or other but I lost the note to remind me."
I think these suggestions may show that the phrase some friend may be an abbreviation of one of these ideas. We leave out the or other.
So the phrase some friend could be just as good as someone. ('some' with a singular countable 'one'.)
I think it may be that we are not letting the listener know exactly who we mean. You're right, it may be that I, the speaker, have forgotten exactly who I am talking about.