Interrogatory responses, like all forms of discovery responses, are not filed with the court. You should just retain the proof of service, in case there is any question about whether the responses were served.
No, discovery does not get filed with the court unless there is an issue or dispute over one of the responses, and then only as part of either a motion to compel or a motion to compel further responses.
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No discovery responses get filed with the court (unless they are exhibits to motions or other small exceptions.)
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Your friend should have served the interrogatory responses by mail on the opposing attorney or party along with a verification (in which you swear your answers are accurate) and proof of service. I'm not sure what you mean by "responses with corresponding docs" but hopefully you are not sending documents in response to the interrogatories. Unless there are appropriate circumstances, you can not answer interrogatories by simply attaching documents. There are exceptions if responding to the interrogatory will be burdensome and the burden is the same on yourself as the requesting party, but you must cite the proper language in the Code of Civil Procedure if you wish to invoke your right to produce documents in lieu of answering the interrogatory.
You should always seek the aide of an attorney in discovery and other legal matters. This response is not meant as general legal advice.