For the sake of simplicity suppose the left branch leads to the ruins whereas the right one leads you lost deep into the jungle.
Case 1. The villager is a liar: in this case, since he knows the right branch leads to the jungle then he'd say it leads to the ruins, but he won't tell you NOW this, so his answer here is NO.
Case 2. The villager tells the truth: he will answer NO again, 'cause he knows the right branch takes you to the jungle and he will let you k now that.
From the above, it's clear you must choose THE other branch if the answer is NO...