there's no definitive answer. she has a solid point. with no memory of past lives why care? this is why the buddha specified that the dhamma is good in the beginning, middle and end. it has direct positive benefits here and now so even if you have no interest in your future incarnations it will still feel the best to practice. tell her to learn jhana meditation and then after she has mastered the first jhana see if she still sees no point in practicing. this is not what the dhamma is about to be clear but it's a solid example of direct benefit we get here and now from practice that does not depend on any belief or care about future incarnations. there are many though, mindfulness feels great and really helps every aspect of ones life, following the precepts will make people love you and care about you, and so on and so on. the dhamma is good for you regardless of future birth(s).