I realise that you probably already completed this section of the module, but will answer it anyway (sorry, just saw the post when looking for some help with the module as well).
I don't think he will be able to easily get around to ambush them at the exit, you could play a bit with the group (you want to take some breaks? Heal wounds? Loot some stuff you missed? Are you sure, he could be after you!). Also Hallod is now tipped off, that people are actually looking for him. Since he feels kind of invincible (due to drugs & his usual encounters with weak villagers) I could imagine him following / tracking the group to ambush them in one of the less inhabited parts of the village with a nice encounter where the players have a chance to detect him.
Since you will most likely have played this already, let us know your solution!