Should my friend stop talking to this guy?

well here is the story. i have a friend named rachel and she has been dating this guy derek for about three years. they had some trouble but, they seem great together now. well a couple of weeks ago rachel started texting this guy jake. they have been texting, calling and sending pics of each other. rachel asked jake to go to homecoming with her since derek can’t. derek is okay with it but, he has no clue that they have so much contact with each other. should i tell her boyfriend whats going on? or should i tell her that she is doing something wrong?