We ran into this conundrum while working on
Crazed Caverns, which led us to banish ALL "split the party" effects from that set. (Don't cry -- it's for the best, I swear.)
The intent is certainly that, while resolving the
Narrow Passage, players in one Dungeon are not subject to the effects, good or bad, of the Dungeon(s) they are NOT in. This does lead to occasional problems, but if you use that as your guiding principle, you won't be far wrong.
In this case, because YOU were not in the DoMM, you could not have a GUAL used to give a monster +10 against you. This answer is provisionally official: I'm going to run it by Steve to get his opinion.