Should academics allow student to submit a possibly flawed paper?
In my opinion, since you aren't the supervisor, it's not your role to stop the student from publishing.  If you are an expert in the subject of the publication and you don't think it's strong, you should advise them not to publish and explain why, but that's as far as you should go.

However, you "let him put [your] name on the paper" -- a paper which, by your own admission, you don't know much about!  Regardless of whether the co-author is a student, and regardless of the quality of the paper, the behavior you're describing is wrong.  In my field, it's actually forbidden by at least one of our professional societies:


  All the authors listed for a paper . . . must have made a significant contribution to its content.


This is generally construed as meaning you should have done a significant part of the research and a significant part of the writing.

Now, violating this rule for a paper that you don't even have a high opinion of seems not only unethical but foolish.

I realize this is a harsh-sounding answer, but you have posted the question anonymously so I hope you don't mind me being frank.