My friend won't back off a taken guy,what do I do?

Okay so I have this friend who is friends with this girl I used to be friends with. My friend is person A and her friend, the one with the boyfriend, is person B. Person A liked the guy first, but person B made her move first. Now person B and the bf are going out and have been for some time now. The thing is my friend can't get over him. She talks about him constantly and gets made whenever they PDA. ike today she went on anther one of her rants. It's bothering my other friends to the point where they feel they don't want to be her friend anymore. She can't get over it and move on. Yeah it sucks but she's haring everyone. I want to tell her something but she's really sensitive and I don't want to hurt her feelings. What should I say?