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The 23-year-old singer reportedly texted his singer ex-girlfriend to urge her to cut down her alcohol consumption and partying.
The 24-year-old was seen drinking beer on the day of her grandmother's funeral and while taking a break in the Caribbean this week as well and smoking suspicious-looking cigarettes.
A source close to Chris, who admitted brutally beating his former partner in 2009, said her is very concerned she will end up dead like Amy Winehouse.
The friend told website HollywoodLife.com: "Chris was really, really worried about her when he heard she was up drinking and partying by herself.
"So he just wanted to reach out to her and show a little support, you know?"
He said Chris had sent her an SMS message telling her to "cut out all that drinking", but Rihanna had not yet replied.
The source continued to explain that there "isn't any love lost between them at all".
He added: "He's just being a man and being responsible, making sure she's all right."
She stoked rumours she could still take Chris back by posting a mystery picture on twitter, showing a handwritten note with the words "still waiting" and a heart.
Rihanna added the caption: "Time heals all, and heels hurt to wait in."