The pair feature each other on two songs released earlier this week, Birthday Cake and Turn Up The Music.
Rihanna, who has been criticised by some fans over the issue, appeared to address them at the Brits when she said: "At times when I feel misunderstood, my fans always remind me that it's OK to be myself."
But Bill Werde, editorial director of the music trade publication Billboard, said Rihanna's decision to make music with Brown could disappoint some of her supporters.
"I think there are people out there that feel betrayed (by) Rihanna," he said.
"She has every right to be an individual... she has every right to date who she wants to date and be with whoever she wants to be with. She's a grown woman. But you just need to recognise that then the fans have every right to feel how they're going to feel about that."
Sandra Ramos, founder of women's shelter Strengthen Our Sisters, says she hasn't seen many abusers change, and that victims - like Rihanna - get "caught up in this cycle."
"She should be working on herself and not allowing herself to be near her addiction, her temptation, who's this guy that purports to be charming when he is a batterer," she said.
Brown admitted assaulting then-girlfriend Rihanna at a pre-Grammys party in 2009.