In the end, if it makes you happy and you aren't hurting people, a person's happiness is MORE of worth than the insecurities of someone else.
It could be argued she isn't happy (she didn't really look it but that could be down to camera/sensing or knowing she could be being ridiculed). She could also be causing herself harm too, in the short or long run. I'm going to assume she has had various tests etc in order to rule out anything physical causing this delusion and that it does in fact stem from a firmly held belief that she has.
The question is how much do you try to address people's mental health or whether you don't intervene on the basis of a lack of harm to herself or others. She does seem to be living a normal life outside of her self-identification, she doesn't seem malnourished, and she seems presentable, social, and well kempt. However these things can change very quickly, especially during a transitory time like her 20s