Dude, stop thinking with your head in between your crotch and start thinkin' with the upstairs head. Think about it logically:
A) She works with you. That's bad enough. If you're working at a real job (vs. burger flipping), this is a formula for trouble. What if you break up? It'll cause bad tension and awkwardness at work and eventually one of you will have to quit for job performance. They say you should be acting professionally and not let personal business affect your performance but the instant you hook up w/ her, you already lost your professionalism. There's a reason why ppl are discouraged from hookin' up w/ ppl at work.
B) Do you really want to hook up w/ a girl who is wishy-washy about her current relationship? If you're thinking about having your own relationship with her, she has to take initiative (not have you tell her) to break off the engagement in order to go out with you. Think about it. If she's willing to hook up w/ guys while she's engaged, what happens when you're the guy she's in a relationship with? Girls like that are only good for one thing - fcuking and nothing more beyond that.
C) If you are an ass and break up her engagement and then break up w/ her months later, you can expect at least 2 ppl to seek you out with the intent of ass kicking - ex fiance who she came running back to and, like the pussy that he is, takes her back unconditionally, tells her sob story and looks for you for messing up their relationship and then there's the dad of the girl. You probably won't be able to imagine the annoying shit he's had to go through. First his little girl is getting married to some guy he doesn't approve of but grudgingly pays for his lil girl's wedding since he's the father of the bride after all only to have some snot-nosed punk make his daughter make another bad decision by both calling off the (already paid for) wedding and making his daughter cry.
I'm sure I probably should've split that last paragraph into more than one sentence but it gets the point across. You're asking for a lot of shit (A, B, C) for little gain (some pussy). If you can't find a decent single girl to hook up with, you don't deserve to get pussy in the first place. Don't be shady. You can be a pimp and still not be a dick.