My DM's saying that since the Help action is found in the actions in Combat in the PHB, he's disallowing use of it outside of combat. It's true that it's found in that part, but I thought you could use it even when there's no fighting happening. Is there a particular ruling for this?
Helping someone outside of combat is under the header "Working Together".
Yes, you can help another player to give them advantage outside of combat.
Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action (see chapter 9).
Being able to help someone outside of combat is still up to the DM's judgment, however. It's limited to situations where a helper would actually be useful, and to where it is possible for the helper to accomplish the task independently.