If she was heavily on board with the plans and Jeremy did this it just highlights what a fucking idiot Jeremy was back in 1985.
Cheating on and dumping a women who knows your secret. How he thought that there would be no backlash from it is anybodies guess.
Don't think him scaring Julie by saying " she would be up to her neck in it with him " would put her off approaching police, friends, relatives etc long term anyway.
She was a 20 year old girl, eventually she would have to unburden herself.