Looks like you might have some aphids/thrips there mate. Try spraying neem oil at 10ml/l every 2nd day. If this doesnโt work you might have to consider using a pesticide worh spinosad as the active ingredient if you arenโt going organic
I would suggest to tell your friend to create a diary and give us more input on the growth. With just this picture, it can be spider mites or it can be a Calcium deficiency or the absorption is scrubbed by a high level of nutrients.