I never said anything to my friend. She was so over-the-top infatuated with him that I felt like she must know best. She ended up marrying him--but, three years later, they got divorced. She realized that he really wasn't the great guy she thought he was. Now, I wonder if I should have spoken up...and if that would have changed anything.
Anyway, back to your question: If they are your good friends, you should be honest with them. It's tricky because people (understandably) can be offended if you don't like the person they're with, and, if you tell your friends, they might get mad or not want to hang out in a threesome. They might even feel like they can't talk to you anymore about their relationship.
So I think it's really important to approach the subject in a gentle, loving, non-judgmental way.
Maybe you could have dinner with each friend and just talk openly about her boyfriend. Start by asking lots of open questions. Ask her how she feels about him, and ask her how it's all going. If you want to get a more critical dialogue started, maybe tell her something that bothers you about your own boyfriend--even if it's something teeny like scary sneezes--and then maybe she will come back with something that bothers her about her boyfriend. Once you've sussed out how serious she is about the guy and talked openly about her concerns (or lack thereof), tell her how much you adore her and respect her and admire her. Say that you know that every relationship has a secret side--so maybe they have a seriously incredible connection that you just haven't seen yet. And tell her that you wonder if this guy is good enough for her (and all her amazingness). Say that you just wanted to check in with her.
Then, leave it. Later, she may want to talk more (for example, maybe she isn't in love with him but is just scared to be alone). You've opened up the conversation in a sweet and welcoming way, so she will be able to explore her feelings and options with you, without feeling criticized or judged. But maybe she really is in love with her guy and wants to be with him. Unless he's actually mean or hurtful to her, you can't really step in too far. It's a tough situation, if you don't like the guy, but if they stay together, as a friend, you have to support her and try to make the best of it, right? I feel your pain, though!
Dear readers, what do you think? Have you ever truly disliked a friend's boyfriend? Did you tell her? If so, how did she react? Or did you choose to stay quiet and let her make her own decisions? Tell me your thoughts!