1. He is calling/texting her a lot
If your man is talking to your friend a lot, especially when you're not around, then something is up. Unless they are planning your surprise birthday party, he is planning to give her a little more ;)
2. He compares you to her
If he compares your attitude, looks, or other attributes to her, stating that she is different, and why can't you be more like her...RED FLAG
3. He is flirty/touchy with her
If he has his hands all over her, and his attention on her, then you certainly don't have his!