What will you do facing a wishy-washy guy?

I was so upset with my sister when she told me that her boy friend does not have a firm stand about their relationship. One day, he told her that he’s so serious with her, but another day, he changed his mind. I suggested her to end up the relation that has never a certain aim. To my mind, his boy friend is a sort of coward, that’s why to leave him soon is the best way to do. Though, my sister can not do that for the sake of BLIND love. Some advices needed. Thank you.