should i be worried?

My best friend (a female) is getting really close with my boyfriend, like they talk a lot together, they text and they even call on the phone. She says that she consider him as her best friend, but everyone is telling me to be carefull... should i? Like now my boyfriend doesn't really text me as often as he used to... and i have the feeling that my friend only text me to see if he's with me so that she can call us... :/  please give me your advises...