A) Quite rude that he won't tell her being an interior designer isn't a proper job?
B) Quite rude that he has told her this?
I'm going to assume B and if I'm correct, he's a joysucking twit. Of course it's a proper job! People spend more money than ever on their homes, it's big business. And it's not just homes, it's hotels, corporate spaces, exhibitions etc. etc.
Tbh even if an 11 year old declared that they wanted to become a yak herder you'd have to be a miserable Mr Gradgrind to tell them to snap out of it because that was an unrealistic dream.