Right now interactions are handled in obj_player's step event, lines 81-98 (the block starts with a comment that reads "//Interacting"). It's a basic collision_circle check so there's two ways you could solve it:
- Hacky way: make the object alter its collision mask depending on how the player is facing, so it has zero collision-able pixels when they're facing the wrong way.
- More stable way: have the interact_script of that object check the player's facing, and simply do nothing if they're facing the wrong way (so you trigger interactions but they only start cutscenes/etc if the player is facing the right way). You might need to make a custom object for these directional NPCs to have an easier time overriding the regular behavior.