Dealing with emails that could be interpreted as being endorsed by you
With a class of only 4 students, the email could be interpreted as being sent by the entire class. Normally in this case, the names of the 4 students would be listed in the email.

For a larger class where it is obvious that the email is form an isolated student the best thing to do is to drop it. Making an off hand comment about how crazy the student who sent the email will only put the professor in an awkward situation.

For a small class, a simple email, ideally just to the professor, but reasonably cc'd to everyone, would be to say that student B does not speak for you.