"She is a true friend."
Translation:C'est une vraie amie.
"Il/elle est" change to "c'est" before a modified noun.
Modifiers can be articles, demonstratives, possessives, numbers, indefinite adjectives...
"amie" is modified by "une", indefinite article, so "she is a" changes to "c'est une".
pourquoi pas "c'est une amie vraie"? Is the word "true" apart of the BANGS acronym ?
It is something you can hear or read with emphasis because "vrai(e)(s)" is essentially judgmental and its placement before the noun expresses subjectivity as the speaker's opinion only. I'll add "amie vraie" as a correct answer but please keep in mind you will find it more often as "vraie amie".