My 16year old wants to live with biological father, what do I do now?

I sent my daughter (16yrs old) to spend time with her dad in Texas for the summer, she was due to fly back on Saturday back to Colorado. I received a phone call yesterday, not from her, but from a lady acting as the middle man between me, her father and her, stating that Meghan did not want to come home, she wants to live in Texas now and I needed to write up an agreement stating that I agree to not put her on the plane and that they will file that along with the rest of the paperwork. If I did not comply and fought him on it, they would fight back and it could get "ugly". I told her to put my daughter on and she said , yes, this is what i want, i do not want to come back to Colorado. By law, do I have to write up some sort of agreement stating that? I talked to some poepl yesterday that said it sounds like they are making sure they cover their tracks and get his childsupport stopped immediately, so it will go on me now. I am a stay-at-home mom, so they arent going to get much out of me like they think they are. My present husband has nothing to do with it, he is not reponsible for her and I think that is what they are thinking. I guess I am wandering, what do I do now. Sit back and wait for them to file papers for turn of custody? I am so heartbroken and confused about all the legalities.