Loosing interest in sudies due to friendship

Hi friends!
My friend's four year old daughter is highly influenced by her cousin sister. She wants to spend most of the time with her. Almost every evening she spends with her and listens to whatever she says. Nowadays she is not obeying her parents and she is loosing interest in studies and other activities too. The parents are worried and being their relative they are not stopping her to visit their place. Moreover she now raises her voice if her parents say anything against her freind. How should they handle this situation without hurting anybody's feelings? Any suggestions?
With Regards
devvrinda