We would def need full story for proper diagnosis and even then i may be guessing…. Sure do with my own plants and rarely works out!
If hasn’t fed anything at all could be water related/bugs/micro deficiencies….way more then i am qualified to diagnose that’s for sure! Also would need to know where it started on plant! Bottom older leaves first its mobile but top down is immobile so that right there cuts the list down as to possibilities but not sure if he shared all that!