That is because if you file ITR-4 you have earned Income under the head Income from Business & Profession.
This means you ought to have a P&L and Balance sheet if you are liable to maintain books of accounts u/s 44AA or if you are not covered u/s 44AA then basic details like Gross Recipts are reqired.
Since your friend has neither filed complete details nor basic details he got a notice.
You are supposed to rectify the return within 15 days of receipt of the notice failing which return shall deemed to have been not filed.