Hypothetically speaking, suppose he met this woman many months ago. And although she currently lived in Idaho, what if she had strong ties to his hometown in Colorado? Maybe she was in his hometown for Thanksgiving weekend?
And perhaps he set up the texts, to be sent a few days later on, when she would be back in Idaho? So all she needed to do, was turn the phone on at some point, when she returned North. He would not have had to totally explain all of the details fully---only that he was in a custody battle and he needed help fighting his mean ex, who was trying to take his child away...
And maybe, the detectives got wind of this girlfriend, and sent an investigator to meet with her and ask what she knew. I am sure they pressured her. She probably came unglued and told everything she knew about the phone, etc.