The 26-year-old is in regular touch with the 36-year-old Harding since she left for the treatment in Utah earlier this month.
“Yeah, she’s good. I mean I wouldn’t really like to go into it, but she’s OK, she’s good,” femalefirst.co.uk quoted Roberts as saying.
Other bandmates including Cheryl Cole are also showing their support to Harding.
“Cheryl is really concerned for her friend Sarah. She made time in her busy schedule to travel over to Arizona and visit her in the rehab centre. She has told Sarah she is more than welcome to use her LA home as an escape pad when she comes out of rehab,” a source told the website.