Hypothetical Question

Ok, this is not a situation I am currently in, per se, but, I have at times been in this situation, as we probably all have. Hypothetically speaking - let's say you have a good friend who is dating someone you are not crazy about. It's not necessarily a personality difference, but rather, you don't think this person treats you friend the way he/she should. For example, let's say right now you're good friends with Rihanna. Is there any way to tell your friend to get out of the relationship? I think I actually said opinionated things to my friends in the past, however, it only yielded unsuccessful results. Hmm.

Ok - two part question: 1) Is there a good way to say something to a friend without ruining the friendship or at least, making the friend uncomfortable for knowing you don't like his/her significant other? or 2) Is there a way to tell your friend that he/she shouldn't be with this person that will actually convince said friend to leave said person!?!?!

I unfortunately have reached a point of feeling confident that the answers to the above questions are no and no.