The passenger identified Jon, and said he fled the scene. If half-naked she said "they" got fucked up and he was driving and took off, that's literally all it takes. That is enough to lay the charges. It's up to the Crown and Defense attorney(s) to hammer out the charges, and the judge to rule on it. Given Jon's past convictions, a leaving the scene charge wouldn't be considered unreasonable.
There obviously would not be call activity between the two while they were together. He only started calling after he ditched her. I'm not certain of the specifics of how the cops ended up talking to Jon, but they know he called her (the passenger) multiple times after the accident.
Somehow there was communication with a police officer, and veiled threats are grounds to get a warrant for phone records. They may not be able to hang impaired driving charges on him (even though he should), but that doesn't mean he still can't face further charges down the road.
Doesn't really matter though, if he did threaten the boys in blue, he's about to make his life fucking miserable, lmao. Jon will be a ticking time bomb for the rest of his days. All it's going to take is one wicked bender and a Tom Aspinall question and Jon's on the hot mess express!