I think it depends on the circumstances. If it's just a case that you don't like the partner, then it's really no-one else's business. If it's the case that the partner has a serious problem (addiction, violence etc) that your friend is perhaps is unaware of then it's your responsibility to speak up, then back off.
Wrong for them how? You must be careful not to let personal dislike or jealousy (yes jealousy) cloud your judgement.
If you know that this 'unsuitable' person is already married, or a wife beater, or an alcoholic gambler, or nymphomaniac (i don't know what gender we are talking about here, so covering all the bases) then if you are really 'best friends' it's your duty to speak up.
Otherwise if you have nothing rational to base your disquiet on, as best friend your job is to be around in case it ever goes wrong, for the purposes of lending a shoulder to cry on, a box of tissues and a bottle of wine, and a ranting session, where you can agree that 'all men are pigs' or 'she's just a tease and you are better off without her'
I told my best friend that she was making a big mistake when she started to go out with an acquaintance of my hubby's when were about to graduate high school. This guy was a liar, cheater, way too old for her, and didn't give a toss for my best friend. My friend was a virgin when she met him, and perhaps it was wrong for her to be overly proud of that fact, for Pig (not his real name, but it should have been) announced to the guys in our circle that he was going to "have" her, and he did, without alot of resistance from my friend, because she was "in love" with Pig by this time. Anyhow, make a long story short, when Pig finally moved on he got her virginity, her Martin Guitar (very nice, expensive model), her heart, and all the money she had. Pig said that he was going away to get their lives started somewhere else, but he would be back for her on her 18th birthday (just a few months away) to take her away into this glorious life he'd described. Is there anyone reading this post who will be surprised when I tell you he never came back? He broke her heart, and her taste in men only got worse as the years went on. We tried to warn her about Pig; I was very specific about what he was doing to deceive her. It put up a barrier between us that finally broke our friendship a few years later. I haven't talked to her for going on thirty years. That's my own personal story. Of course you must be honest with your friends, but you must also be prepared for the consequences that may happen. Do you love them enough?
I agree that if I was asked my opinion I would give them the honest truth. If I am not asked then it would just depend on why I thought the person was bad for them. Like they are addicted to drugs or an abuser, something like that then I would offer my opinion without being asked and hope that our friendship would survive.
tell your friend the truth. say "you know I am your friend, but it concerns me that so and so may be...." If she/he disagrees, I would say "I am only looking out for you because I care about your well-being"
Oh wow! I'm having the same problem! Or I've had the same problem. He was my best friend, actually.
To answer your question, YES! I would definitely speak up.
When my best friend dated another "sorta" friend of mine and his, I told him straight up that she was all wrong for him. She cheated on him and I told him. He got upset that she would do that to him and thanked me but of course, sometimes having feelings for a person you like can overcome the relationship between friends and he told her and we all got into an argument. She forced him and threatened him about the friendship he and I had together and he ended up leaving me as a best friend. We no longer talk because of her. I don't regret telling him about how and who she really was. But I hate that I lost him. He was the greatest friend I had.
Now, they've been together for a year and a half and she moved hours away where he's going to school. Next semester they're moving in together.
On the inside, I wish that they break up so I can have my best friend back, but honestly, if he's happy, even without me, I'm happy for him.
Really, it's a hard thing to do. If you tell him or her, there's a possibility that everything could go wrong and you could lose having that person as a friend because of his or her dumb "significant" other. If you keep your mouth shut, then you could potentially witness your good friend get his or her heart broken.