Can I file an RTI application on behalf of someone else, like a parent or friend?

An RTI application must be made in the applicant's own name - you cannot sign it for another person. But you can ask about any matter, including one that affects a family member, because the RTI Act does not require you to have a personal stake. The practical limit is Section 8(1)(j): personal information of another person can be withheld unless a larger public interest applies. So the cleanest route is for the concerned person to be the named applicant (you can still do all the work), or for you to frame questions around records and processes rather than another person's private details.