For me it's a bit of both. He seized the opportunity in a situation where he thought she'd be unable to fight back, he had an audience. It was manipulative and I do think he'd rehearsed it in his head a few times, he barely paused for breath and I don't think he even stumbled over his words once.
On the other hand, he was expressing a view many of them share but didn't want to say, wouldn't dare say. Whether he did it to make her change her ways or simply to appear the hero, or even to just get it off his chest, he knew he had a captive audience and less chance of a fight erupting.
Was it harsh, yes it was. It was his opinion though and apparently being frank and 'telling it like it is' is what many of the housemates want and what many of the viewers want too, judging by recent support for some very 'in your face' and vocally provocative housemates.
Is it Bobby's job to tip-toe around Farrah because she has 'issues'? No. Is it his job to try and change her, no. However, it is his right to express his opinion - especially as he'd been invited by BB to be frank, he certainly was that!
If he thought she was heading into the Diary Room to say she'd had an epiphany and would start to consider her behaviour and her fellow housemates, tone her behaviour down, he would be wrong.
She doesn't think she's done anything wrong at all, not sure Bobby does either - and it's up to individuals to decide if they agree with that or not - but he at least was aware that he may have gone too far, even if only because it could affect his duration on BB.