If she doesnt do WC the betta will get sick and die. Trust me I speak from experience. The filter can do most of the work but she needs to do her part and be more responsible for an animal that she bought. If she doesnt want to do WC then ask her if you can take the betta off her hands. IF you have a betta already in your big tank, You may want to keep her betta alone in his own tank. Just do WC and testing to make sure everything is stabilizing. Without WC the Nitrates increase and the betta becomes susceptible to illness and disease. In Short...ask her to give you the betta because it needs proper care. Good Luck.