Anonymous wrote:No. Your time to speak up has passed...
So why is there a place in the traditional wedding service for people to stop the wedding? But seriously, I think it depends on how specific your worries are. My friend became engaged to a very nice guy but they didn't seem to have any chemistry. How in the world do you talk about that? Sure enough, down the line they divorced, but I don't think there was anything I could have done to intervene.
That said, I would definitely want to say something if they are bigger warning signs. At the very least, if he's a good friend, you can ask him questions. "So, you're getting married to Larla? Tell me more about her." Or, raise specific concerns. "You seem kind of stressed out about Larla's family/ex/lack of job/mean jokes. Is everything OK?"
As the others have said, prepare for him to ignore you and possibly get angry. But I think if you ignore your own concerns you're not that good of a friend in the end.