Unless it's a clear hazard, I'd say nothing unless asked, then be polite but not enthusiastic.
This! If they are really being a jerk, you know they're cheating or trying to, or you googled them and found out they have a warrant for their arrest... Tell her! If they just don't seem right, you don't have to lie, but try to stay neutral.
One of my best friends is the queen of this. Unless there's a specific reason you don't think it's a good idea or it wont work out. Just grit your teeth and ignore it, I know how super annoying it is but chances are they wont listen to you anyway, or they may even get upset. Everyone has to make their own mistakes sadly.
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.