depends how long she has been dating him. maybe she is interested in someone at work.

Does she says she's single and looking and flirt with co-workers or just that she's single as in unmarried? And do you know if it's an exclusive relationship or not?

single and flirt with co-workers been dating for a few months and i believe they are exclusive

So you don't know that they are exclusive. Unless you know, I wouldn't assume.

because you two aren't really dating. you are stalking.