that is, i don't think "bitch" is one word. it's a series of homonyms. the etymology thing is kind of whatever. but "you're a snivelling, manipulative whiner" and "you're a bossy, dominant jerk"...these are not the same accusation. they're the opposite of each other. and, while one is sexist in just about any context, sometimes the other is just an accurate description of somebody you don't enjoy spending time with.
then, there's the use of bitch as "friend". well, it's not my vocabulary. but it's really pretty harmless. it could just as easily mean "enemy". and, why waste time wondering if you're being nice to your enemies?
the lack of any real meaning to the word really makes any broad, general analysis pretty much impossible.
that said, it can be used in a brutal, misogynist way. but so can anything. you can imagine some violent douche calling his redheaded victims a "carrot". "that fucking carrot deserved it". i'm not making light of the situation. but, when you substitute like this you can see that the description of the subject in such a situation is actually the least pressing part of the issue.
should rihanna be talking like this? well, you could ask her, but i bet i know her response.
"please, bitch. don't you fucking tell me what to say."
fwiw, rihanna did not write the song. and, i'm left wondering if she's actually the topic of it.