You may go ahead and file the PIL so long the issue is related to public interest. The information obtained via RTI should be material to the petition. Who obtained the information via RTI is immaterial because information obtained via RTI is anyways public information and legally admissible as evidence.
If you have still doubts, file another RTI Application in your name and seek certified copy of Letter No.......dt....provided to RTI Applicant Shri....................as information for his letter No.................. This becomes official copy to you, without any doubts. (Infact this is not necessary, but one HC insisted that concerned official should also be examined while admitting a public record)
I agree with Democratic Indian. While filing PIL, please ensure that the matter is really concerning the Public and not some personal issue is involved. Now a days Courts are coming on the PIL petitioners if such PILs are on flimsy and irrelevant grounds.