You're not always going to like everyone your best friend dates, but usually you can let go. Sometimes, however, there are the relationships that are just totally unhealthy.
Things you will need:
First, take some time to question why you don't like your friend's boyfriend. Does he treat her badly? Do you find yourself only ever receiving phone calls from your friend because she's sad about him and needs cheering up? Is he cheating on her? Whatever it is, take time to really get your reasons aligned.
Talk to mutual friends about how you're feeling. If you're the only one, maybe there isn't something wrong with the boyfriend, and there's something else that is causing the problem. In that case, you'll need to take a different approach to this. However, if you find that most of your friends agree with you, it is time to have a talk with your friend.
To have this discussion, you must decide how many people should talk to her. If you\'re worried she\'ll feel as though she\'s back into a corner, maybe you should do it yourself. However, if you feel that you would be better off with a group of people who can reinforce what you are feeling, then get a few close friends involved.
Whatever you do, talk to her in person and not over the phone or by e-mail. Approaching her in person will show how strongly you feel about this subject.
You need organize a time to get together. Before arranging the meeting, don\'t keep your friend in the dark about what you need to talk about. Make sure that she knows you\'re going to have a serious conversation so she\'s not thrown off when you talk about it. Decide where you want to have the meeting based on your friend\'s emotional level. If she\'s more likely to throw a fit, have it at home. If you know she\'ll be somewhat emotional, but stable, do it in public so she won\'t go any further.
Now it's time to talk. When you see her be friendly, but stern. If this relationship is really unhealthy, you're helping her for the better. Sit her down and tell her exactly what is wrong with no "I think"'s or "maybe"'s. You need to be completely positive (i.e. "You are in an unhealthy relationship and these are the reasons why..."). If you're in a group speak one at a time and don't overlap each other.
Speaking up to a good friend isn't easy, and even if she doesn't immediately break up with your lousy boyfriend, sometime in the future, she'll know that what you did was the best thing for her.
Tips & Warnings
Don't be afraid to be honest, just don't be harsh.
Make sure your reasons for not liking the boyfriend are legitimate and your opinions are backed up with support.