You know, in the big picture--by the time the friend listens to you they are going to be ready for the next one to show up. I have a friend that is ready to have his new girlfriends move in and talks permanent issues with every new girl in the picture--makes for some long days for hubby and I when we have to listen to him....but he gets over it and goes on.
After talking to my hubby, we realize he is just desperate for someone to love and love him back. It is actually sad and lonely for him--so he overcompensates in the relationships.
Appreciate what YOU have and tolerate their raves about the new guy. Hopefully they find someone to love and be loved by, but until then just be the best friend you can be. You don't have to lie or play the game--just say you are glad they are happy.