She seems like a good friend, but she's actually up to something bad. She wants to secretly sabotage your relationship with your boyfriend so that she can swoop in and win his heart.
 is up to something bad or evil has bad or evil intentions指一個人有不善良的或邪惡的目的
她看起來是個好朋友但其實不懷好意, 她想搞壞妳跟妳男友的關係, 這樣子就可以趁機介入贏得妳男友的喜愛.
When a man and a woman start dating, they need to go through an ajustment period before they decide to marry each other.