Have your friend sign the end of the letter ,or clarify about
what is her/his status or job in that country and also say
that they don't have any attention to let you to overstay or
try to do anything illegal in that country.
My sister got the US. visa because she got a very nice
letter from her brother in law that wrote a very good letter,
and my sister said she got it because of this letter. (she doesn't
make much and still single) .
I think the invitation letter is one of the most important thing to
make them give you visa , make sure they put address,cell phone,
home phone and job in the letter.