"He is her friend."
Translation:C'est son ami.
'Il est' becomes 'c'est' when followed by an article or a possessive (son) + a noun (ami).
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.