It may not seem like proof on the front of things, but it gives the LE reason to suspect based on if and why he lied about his whereabouts on 3/5/07 in his original statement - and there is an original statement if he was questioned in 2013. He obviously gave himself an alibi and was cleared of suspicion. Then in 2017, a “friend” comes forward with information linking CB to MM, he tells them what he heard and what he saw. He also links CB to an unsolved rape in the same resort at this time, and this information turns out to be factual. So they have enough cause to believe that what this friend told them about CB and MM is true also. Phone records put his phone in PdL. His original statement might say “I wasn’t in the area, my girlfriend had my phone” and that’s why they mention that specifically. But now they have the statement from the friend, they have enough cause to believe he was lying. This friend has obviously told LE how he killed her, as that is what they have said they are so certain of. Based on the friends true statement linking him to the rape. They just need the other person from the call to come forward and confirm it wasn’t his girlfriend using his phone that night and they can prove he lied in 2013. Why would someone lie about their whereabouts if they had nothing to hide?