do i try harder to bust it up or do i sit on my intel and wait until they finally meet for PA and bust it then?
How iron-clad is your evidence right now?
Your question strikes me as "I have evidence that the pyromaniac has matches and kerosene in his possession; do I wait for him to burn down a building, so I can catch him red-handed?" Reasonable people can disagree about exposure, but if you're going to do it, then do it -- there's no reason to wait for things to go PA. How would you feel if she caught as STD from such an encounter, and you hadn't done everything you could to stop it before it got to that point?
