I think she should thank you for buying a bass for her but it sounds to me like she wasn't serious when she said she wanted to learn bass. Have you ever heard people say "I'd like to speak French" or "I'd like to learn to ski" but these are passive statements? It means they would like to do that but it's not a priority or something they intend to do right off. Alternately if you asked her, she might not have wanted to hurt your feelings. This sounds like what your cousin is doing. Best wishes.