The t-shirt has been on sale since the end of last year. And when Ri-Ri found out about it, she and TopShop negotiated for eight months regarding the rights to her image. But all hell broke lose when Topshop offered the popstar a low $5000. I'm guessing that's when she hit them with the "hell naw" and sued them.
But the true question is what issue will Rihanna plea? In the U.S., this case is a common 'Right to Publicity" case. This case arises when a person's image, person, or personality is commercially appropriated without that person's permission. This happens to celebrities and other famous people all day everyday.
HOWEVER, the U.K. does not have such a lawsuit. WWD reports that in the U.K. "anyone who legally purchases an image can use it, unless it is trademarked." So in order to sue, Ri-Ri's lawyers would have to prove that the Topshop T-shirt portrayed her as "endorsing" the brand.
Rihanna already has a fashion line at Topshop's competitor River Island. So we can count them out as a viable duo for a future fashion collaboration.