It’s normal in English, although it was originally borrowed from French: https://en.oxforddictionaries.com/definition/brunette
The English word for "brunette" is borrowed from French. So is "blonde". English uses several words from French that have made their way in to English-speaking dictionaries. Other words such as cliché, liaison, and cul-de-sac are just examples of words we never bothered changing from French.
What you seem to be asking is if brunet/brunette (still a French/English-borrowed word) is determined as masculine or feminine. Brunet is not the English form of brunette. It's simply the masculine.
This particular question asks if she is a brunette or blonde. If we were speaking French, it would be a word likely ending in the letter "e".
However, English has accepted the feminine to represent any male who is a brunette or blonde also. Though, commonly people resort to a man being "blond" and a woman being "blonde" - for example. There is no right or wrong in English where the masculine and feminine are concerned as both have been accepted to describe both men and women. Only those who want to find the connection in French will be stuck on blond for men and blonde for women.
I myself, am blonde. I prefer to spell it with an "e". I like the look of it, and still equate it with femininity. But there are professionals who write for a living who will group all "blondes" as "blonds" and vice versa - and neither one of them would be incorrect in English. French, however, is a different story!