I'm not sure what the big deal is, tbh. She was being polite. Then there's the matter of Rihanna. People do like her and she's a pretty big deal these days. If she wasn't back with him then he wouldn't be at the Topshop party or in the front row at the Grammy's. But he is now her boyfriend and it's polite to be polite to your friends boyfriends! I'm sure it makes a lot of people uncomfortable, but that's the way it is for now.
If you knew Rihanna really well then it would be acceptable to offer some advice on what a douche she's dating (Jay-Z has tried that, hasn't he?). But the Grammy's wouldn't really be the place!