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Q1: It is not legal to carry prescription medications for someone else.
Q2: They're asking you to not tell Customs because they know or suspect that Customs will confiscate the medicines.
Being caught with medicines not your own may also result in your being charged with the criminal offenses of smuggling and/or lying on your Customs Declaration. Any such discovery by Customs will make future entries by you back into India significantly more difficult. None of this would be enjoyable.
All in all...just say no.