Another Hypothetical Question

Recently - I had a close friend ask me, before introducing me to her new boyfriend, to be completely and totally honest with her about my opinion of him.

Ok - question - when the opinion is negative  (not necessarily specific to this circumstance, but hypothetically speaking) - what CAN be done?  Can a person EVER tell a friend she doesn't like the boyfriend or girlfriend without doing anything but damaging the friendship?

That being said - I can honestly say I've sat and watched several friends date people who, at the very least, did not treat said friends the way they deserved.  I've almost always kept my mouth shut.  Why?  Because of the reason stated in the former paragraph.  Said friend would just push me away, future situations with said friend and bf would be uncomfortable, said friend would begin to censor the significant other's bad behavior, etc.

That being said - I'd like to think if I was in an unhealthy relationship, my friends wouldn't stand idly by and turn a blind eye.  I also know that if I were to date someone and a friend made a disparaging remark about the individual, I would be annoyed based on the fact that I'd witnessed said friend put up with someone less than ideal, so there'd be a hypocrisy factor involved.

What should be done in these situations?  Your thoughts?