As per Schedule 1 r.w. See Regulation 5(1)(i), NRI are permitted to open/hold/operate NRE Accounts.
FEM (Deposit) Reg 2000, Regulation 2 (vi) defines term NRI.
Regulation 2
(vi) ‘Non-resident Indian (NRI)’ means a person resident outside India who is a citizen of India or is a person of Indian origin;
(xii) ‘Person of Indian Origin’ means a citizen of any country other than Bangladesh or Pakistan, if
(a) he at any time held Indian passport;
or
(b) he or either of his parents or any of his grand-parents was a citizen of India by virtue of the Constitution of India or the Citizenship Act, 1955 (57 of 1955);
or
(c) the person is a spouse of an Indian citizen or a person referred to in sub-clause (a) or (b);
If u/ur customer is satisfied by above I can suggest more indirect ways if ur customer is holding OCI/PIO Card as per MOA Scheme.