Tell him it was an accident but make sure he knows it made you uncomfortable. For example, say 'I know it was an accident, but it's not something I should see. Let's just forget it and be more careful in the future.'
Tell him calmly but firmly that this is not okay. Explain that you respect him as a friend but such behavior crosses the line. Hopefully, he'll understand and apologize, and you can move forward with the friendship.
You could also be honest with your friend and tell them that you don't want to do it because you value your friendship too much. Chances are, they'll understand and respect your decision. If they don't, then they might not be as good a friend as you thought.
Talk to your best friend about it privately. Let them know what's going on and see how they feel. Maybe they also don't want this to happen. You can then decide together how to deal with the dare, like coming up with a different 'punishment' for not doing the kiss.
This is an inappropriate and uncomfortable situation. You should firmly tell him that this is not acceptable behavior. Set clear boundaries and distance yourself from such inappropriate actions.
Tell your friend firmly that this is not acceptable. Explain that it's wrong on so many levels. If she's a true friend, she'll understand. You don't have to be mean, but be very clear.
You could play it cool by making a light - hearted joke about it. For example, you could say 'Whoa, that was a close call for my butt!' This shows that you're not overly offended but still makes it clear that it was a bit of a misstep. Or you could simply and calmly say 'Please be more careful' in a friendly tone. The key is to handle it in a way that doesn't make the situation more tense or uncomfortable than it already is.
First, you need to have a calm and private conversation with your wife. Ask her why she did such a thing and express your feelings of discomfort and disappointment. Then, you may also need to have a talk with your friend, clearly stating that this behavior is unacceptable. Finally, consider seeking professional marriage counseling to help resolve the underlying issues in your relationship.
He should just leave quietly and quickly.
Apologize if you think it made them uncomfortable. Then, establish some boundaries for future situations to avoid any misunderstandings. For example, you could say that in the future, you'll make sure to find separate sleeping arrangements.
Don't keep secrets. As soon as you realize his feelings, talk to your best friend. Be honest and straightforward about what's going on. She may be hurt, but she'll be more hurt if she finds out later that you knew and didn't say anything.