She stole her best friend’s boyfriend (from her)

The most obvious possibilities are that (a) her best friend is a "her", in which case the parenthetical is redundant and one could delete it, and (b) her best friend is not a "her", but a "him" (or something else not a "her"), in which case the parenthetical is not redundant but essential. If we assume the author knows how to write, he or she has added the parenthetical to make it crystal clear to the reader that her best friend is a "her", and not a "him" or something else.

The phrase is taken from a Spanish bilingual dictionary, the parenthesis is probably used to clarify to Spanish speakers who the "best friend" refers to, in this case a woman.

In English "friend" is gender neutral, but in many Romance languages it is gendered, e.g. in Italian it's amica (f), amico (m), and in Spanish it's amiga, and amigo.