Amiga is Spanish for female friend.
Amiga in Portuguese is female friend, so you see it's not the Spanish for female friend, rather the portuguese as well the spanish for amiga friend.
Also the accent does not belong either to US, and the reason they speak it with husch accent is due to the phonetic limitations present in the native language, English.
So obviously, Amiga should be said as it was meant, with either spanish portuguese accent, being those two virtually undistiguisible (as far as the word Amiga goes). As such, it's said as it is written... Amiga :-D
