Sometimes we have people in our lives that we use for comfort. They are just easy to be around. I think Jordan and Daisy are just people who use each other... that was part of the culture in the 20s. Here are some reasons they are not likely true friends:
1. Jordan seems to know that Tom has a woman. She's nosy and wants to see Daisy catch him, but doesn't seem to reveal any evidence of helping Daisy or telling on Tom what she knows about it.
2. Jordan expects Daisy to wait on her while she stays at Daisy's house. When Jordan goes to bed in chapter 1, she asks Daisy to get her up at 8, and Daisy says, "if you'll get up." This suggests that Daisy doesn't care too much for Daisy.
3. Jordan is just arrogant. Nobody wants to be friends with someone like that. Jordan knows she's attractive and she plays a professional sport so she expects all people to treat her well.
4. Jordan goes to parties at Gatsby's without taking Daisy. It seems as if she goes to the parties regularly. If they were such great girlfriends, this would be a regular activity they shared.
5. Good friends take you up on the advice you give them. Daisy seems to try to get her to be with Nick. That transpires on a friendship level, but we do not see evidence of a relationship budding.