A friend of mine has found me a position to apply for in the place that he works, but it's not really being advertised or anything just yet.

He's given me the address of the HR woman (who he spoke to about it) to send my CV and a letter to to apply for it now.

What's the best way to begin this letter, I want to make it clear that I'm the person my friend spoke to her about (as he said she was impressed with my experience etc) but I'm not sure how to do this.

Can anyone help? Thank you :)