HRA deduction

HI one of my known have submitted wrong pan by mistake for hra benefit last year to employer and got tax benefit and same is reflecting in his HRA. but now he donot want tax benefit as pan is wrong and the pan owner won't show this income in his itr which might cause trouble for him. Can we remove this exemption and pay additional tax subtracting HRA under section 10c. do we need to inform employer or simply pay tax as he has to file his return for last year.