Her friend wasn't in on it. I think they had her in there just to make it more convincing that she had this high end job, her coworker friend was covering for her at work so she had to explain some job related technicalities to her over the phone. When she later got frantic and told her friend what was happening, her friend was genuinely concerned and alarmed. It was really sloppy, lazy writing that they never had the friend trying to phone her back, especially when she told her to keep checking on her and send help if she didn't hear from her again.
When you get up in the morning, how do you decide what shade of black to wear? (Shallow Grave)
she told her friend to call if she hadn't heard from her or show up (something like that) but the friend never reappeared or tried calling.