Okay, real quick before I peek @rodan’s answer … I think 男をダメにする彼女 is one of those culture-specific concepts that is so well-known that people reference it without need for any further context. (I’m not sure I realized that the first time I read it.) Based on the things that came up when I googled that phrase (eg), I think the idea is that a girlfriend spoils her boyfriend to the point that he becomes a ダメ男. So in this case, the main character is just choosing a kind of silly and romantically-tinged phrase to say that her friend is spoiling her. (To which her friend essentially tells her she is already ダメ, but also in a kind of silly and romantically-tinged way.)
After reading @rodan’s answer … I think we kind of said the same thing, aside from that rodan addressed the entire situation, whereas I was focused on the one phrase that I initially found a bit confusing. (I think considering a broader range of things when answering language questions is one of rodan’s great strengths. 
)
P.S. that was a really fun one to reread as well!