"So" and "such" are in the lexical category of intensifiers. This means that they both express a certain idea of emphasis, intensity, or emotionality. As a general rule, "so" applies only to adjectives and adverbs ("I’m so glad to see you", "the day went by so slowly") while "such" applies to nouns ("my neighbor’s dog is such a nuisance"). DO NOT mistake the one for the other! You shouldn’t write "my friend is such beautiful" nor "my friend is so a beauty". It is either "my friend is so beautiful" or "my friend is such a beauty".
Also, don’t miss up with the place of determiners when you have an adjective and a noun together. You can normally stress either of both, although stressing the noun is more usual. Let’s imagine that you want to say that your friend is really an extremely beautiful girl. The commonest way to shape your sentence in English would be to stress the nominal phrase, using such: "my friend is such a beautiful girl" (careful: the indefinite determiner comes after "such"). Never write "a such". It is always "such a". You may choose, in rarer cases, to stress the adjective with "so", but then you need a syntactic inversion: "my friend is so beautiful a girl". However, such a phrasing is old-fashioned, and found mostly in highly literary and poetic texts.
