My POV on Rubina buttering Salman is that he said something offending/hurtful. She expressed her displeasure. But rather than listening to her Salman ignored and everyone was laughing. To be honest it is totally possible she realised how moody and a hell of a bully Salman is. Only way to protect herself and her husband from his taunts would be to be in his good books
Hence, on the spur of the moment she tried to do that cute stuff and we all know salman khan does have some soft corner for " cute girls " and though I have not watched the episode I am sure after those antics salman would have been relatively calm towards the couple in the rest of the episode.
Now, why Ruby complained later? Because I guess she very well realises how people bitch in the house especially Nikki and gang( they only target the couple tbh) and I guess she must have been hurt how her husband was mocked on national television by a so called "big star" and everyone else who just wants to be Salman's good books can take up that topic and keep bashing Abhi( which they definitely will do). I understand her fear and she is not wrong. All this once they get of the house will be seen by both Abhi and Rubina and most people they know and can definitely affect the dynamic of their marriage.
Her action according to me is commendable and I hope colours and Bigg Boss respects her emotions even though this is the first time someone stood up against Salman.