Which one would she rather study?
Both have the potential to be part of future problems and be in demand, the jobs required and that will pay well are not invented yet, so being adaptable and lucky will also have an influence on future earnings.
As she knows there is an explosion of mental health diagnosis recently, I predict, there will be a huge need for psychologists for the adults of the children today whom have been brought up on screens, shut off from other humans. Some damage that will take time, patience and expertise to improve.
With biology, living things matter, they form the basis of the building blocks of life, as human actions mess with the evolutionary balance again adapting or slowly down the change or trying to predict what will happen with unrecoverable damage, experts will be needed.
So either will be part of the future.
Lucrative? Who knows, who knows whom is going to pay for what.
If it was one of our children asking this question, between these two subjects, they really couldn't decide, wanted to try and future proof their career, I would take earnings as such out of the equation, for this decision, I would veer a little bit more towards biology, but if they chose psychology would try and advise them on the choice they made.
She should study what she wants to for her BSc.