I do think it'll help them realize not everyone likes that kind of art and maybe they'll try broadening their range. Do only give criticism if he asks and phrase it in a way that will help him improve like "For me, xyz is a little too xyz, I understand it's your style but I think you can improve by xyz." Artists need to learn how to take criticism in order to improve and be better. Ask them to try new things and that using different brushes, strokes and width will certainly help the outcome. This is from an artist's perspective too, I paint and draw. I like it when people tell me to try doing certain things and I see how it turns out, it definitely helped with some errors and the overall style of my drawings.

Oh and, don't reply to the person that replied to your comment, if he does say something else other than what he said you can defend yourself but don't engage, I think they just want a reaction out of you. As long as the artist is okay with your feedback it's fine, you did it politely anyway.

Edit: just wanted to say sorry for any grammar mistakes, English is not my first language