fickle

adjective
1
changing often and suddenly
دمدمی‌مزاج, بی‌ثبات, دمدمی, ناپایدار
2
often changing their mind in an unreasonable way so that you cannot rely on them
دمدمی‌مزاج, بی‌ثبات, متغیر
She had been a fickle friend, even at the best of times.
another example of his fickle behaviour