Student or Learner
1) Since when is he your friend? Correct it, please
2) Please put this printer back to its place. correct it please
Thanks in advance for correcting sentences!
Since when has he been your friend?
1) Since when is he your friend?
I hope the previous respondents don't mind if I try and clarify this for Twilit.
The phrase as you've written it is, as others have mentioned above, perfectly good colloquial English but it has a very particular meaning. It is really saying, "I'm not sure he is truly your friend". It is skeptical.
However, if you are just asking for basic information about how long he has been a friend then you would say something like, "How long has he been your friend?", or "When did he become your friend?".