Bad idea? Helppp

So I'm 18 years old. I lost my virginity to this guy when I was 17 no names mentioned so I'll call him "Bob". Well me and Bob have been "friends with benefits" off and on for about a year now. Bob is now living a couple hours away and only comes down every now and then, but when he does the sex is great. Bob has a friend who lives here that I've been attracted to since I met him, and again no names mentioned so I'll call him "Zack". Well me and Zack are not doing anything serious but we have kissed and played around a little bit. Bob does not know about this, and I don't think he'd have a right to be mad. I feel like now that Bob is living a couple hours away that Zack would be the right guy to help me take my mind off things. So pretty much what I'm asking is.. If you were a Bob and you were having sex with this chick and then you moved away and she started having sex with Zack, one of your good friends?

How old is Bob? Why did he leave you behind?

Bob is 20. And I guess he just wants to get his shit together and make something of himself. I don't think he "left me behind" all we were just fwb and hung out with the same people.