The British designer - who created Team GB's Outfits for the London 2012 Olympics and Paralympics - reportedly frequently calls up the 24-year-old pop star to talk about her latest style inspirations because she wants to reach out to a younger generation of fashion lovers.
A source told the Mail On Sunday newspaper: ''They have been having regular chats. They call them 'mood calls'.
''Stella rings Rihanna and asks her to explain what has influenced her mood that day - colours, textures, patterns.
''Stella feels she needs to tap into a younger generation and Rihanna is happy to help. It's a bizarre way to create a fashion line but they work well together.''
Rihanna is currently the face of Sky Living reality show 'Styled to Rock', which sees aspiring designers pitted against each other in a bid to create a festival look for the 'You Da One' singer.
However, the trendsetter's show - which boasts Girls Aloud star Nicola Roberts and designer Henry Holland as judges - hasn't been as successful as predicted. The first episode was watched by 95,000 viewers, before falling to just 79,000 with the second.
She recently commented: ''The fashion kids always do something new, always do something innovative and that's what this competition is all about, and that's why I came to London.
''I really wanted to do this show because I love discovering new designers and finding things that are fresh and trendsetting.''