The Scottish DJ called time on his nine-month relationship with the 'RIP' hitmaker earlier this month after their clashing work schedules put a strain on their long-distance love.
However, he flew to Vancouver, where Rita is currently filming for 'Fifty Shades of Grey', this week to try and win his former beau back.
One of Rita's friends said: ''Calvin is pulling out all the stops.
''She was impressed when he flew out to Canada to be with her, as she knows he's really busy right now. They had lots of fun. They had day trips together.
''Calvin explained to Rita that she's very important to him, and that he just can't see where things have gone wrong between them.''
The 23-year-old singer is yet to agree to give their relationship another go, but Calvin, 30, is refusing to give up on her.
The pal told the Daily Star newspaper: ''Rita isn't convinced yet. She knows Calvin is a really good guy. But she's really struggling to keep her attentions focused both on her career and their relationship.
''She finds it so hard being in a long-distance relationship, and their schedules seem to constantly clash. But Calvin had told her that he sees her as his lady.
''He'll do whatever it takes and he's not going to let her go without a fight.''
Her parents weren't too fond of the idea.
The flat at Hauptstrasse 155 was where Bowie and Iggy lived between 1976 and 1978 in the city, which inspired the so-called 'Berlin trilogy' albums.
Grint will star alongside Dougray Scott and Ed Westwick in a 10-part TV series for Sony's streaming platform Crackle.
The Brits teamed up at the Hollywood Rose Bowl to perform a cover of The Purple One's 'Nothing Compares 2 U'.