Crystalake realises that…
By accepting funds from her boyfriend, no matter how innocently offered, she would automatically have been giving him a kind of upper hand. The relationship would not be an “equal” terms anymore.
As the lower salaried one in their relationship, her choice was either to assert her independence by admitting she didn’t want to pay beyond her means, or to allow him to pay, thereby showing a reliance on him for which she clearly isn’t ready.
As always, any row about money is about far more than what you can afford.