Writing a formal letter in someone else's name

How should I begin a letter if I'm writing a formal letter to find out something for someone else's application

Dear ...

I am writing on behalf of ....(person) to ...(purpose)

Or you may want to introduce yourself first [name, address, etc]. Then..

I am writing this letter [or correspondence] upon [friend's name] request/suggestion...

Or as the other poster had written.. In behalf of [friend's name], I am writing this letter to..

But don't forget to introduce yourself first to add validity and strength to your letter.

Hope this helps..;-)