Someone PLEASEEEE help me with this pleaseeeee!?

this guy told me i like him and i told him i like him (Mark) back too and were planning on going out on our first date tomorrow. but then this girl (Cindy) that both of us knows just recently broke up with her boyfriend and started venting to Mark. I dont actually remember how she brought it up but she said shes starting to like Mark and keeps talking to me about him everday, but she doesnt know that i like him and him likes me back and our flirting on fb makes it seem pretty obvious xD. i sort of want to tell her but me and mark arent officially going out yet so how am i supposed to cope with this? oh yeah Cindy is a really nice person but i dont know im getting a feeling that shes crushing on Mark inorder to forget about her ex. so what how am i supposed to react to this and stop her from talking about him to me everyday?