That was quick! Thanks. But if somebody says something that really bothers him and he looks for a solution to his problem but his friend doesn't know the answer, can he say 'who wants to know' meaning 'i wish i knew'?
That doesn't make sense to me, no. "Who wants to know?" is a question, not a statement.
It's used in BrE a little sarcastically sometimes. Someone might say to you:
"How much do you earn?"
Your reply could be "Who wants to know?" What this means is that you're interested to know who actually wants this information. Is the person asking you for themselves? Are they asking you on behalf of someone else? Why is it anyone else's business how much you earn?
I have just thought of a similar phrase which could be used to mean "I don't know and (perhaps) I wish I did". That phrase is "Who knows?!"
What's the square root of 364,251?
Who knows?!!!
Why do so many people think Tom Hanks is a good actor?
Who knows?!
It's more of an exclamation than a question and it sort of means "I don't know and I can't imagine that anyone else does".