"He is her friend."

Translation: C'est son ami.

[BlackSea]

And what exactly is wrong with "Il est son ami"....?

[simon.currie]

'Il est' becomes 'c'est' when followed by an article or a possessive (son) + a noun (ami).

[Melinda82]

We use "C'est / Ce or C' " instead of "Il est/Elle est..etc" when the noun is preceded by the followings:
indefitine article (un, une, des);
definite article (le, la);
possessive pronoun (mon , son,notre..etc).
e.g: C'est une femme.= She is a woman.
C'est mon fils. = He is my son.
C'est notre homme. = He is our men.

Besides, in French they always use "C'est" before names! :
e.g. C'est Luca. = She is Luca.